THE 


, 


STUDY  OF  ROCKS 


AN  ELEMENTARY    TEXT-BOOK  OF  PETROLOGY 


BY    FRANK    RUTLEY,    F.G.S. 
u 

H.  M.    GEOLOGICAL    SURVEY 


LI  UK  A  \\ 


\ 


D.    APPLETON    AND     CO. 

NEW    YORK 

1879 


EARTH 

SCIENCE^ 

LIBRARY 


PREFACE, 


THE  rapid  advance  of  Petrological  study  during  the 
last  few  years  has  rendered  it  imperative  that  some 
English  text-book  should  be  written  for  the  guidance 
of  students  in  this  branch  of  science.  Several  good 
manuals  of  petrology  have  recently  been  published 
on  the  Continent ;  but,  hitherto,  comparatively  little 
has  been  done,  in  this  country,  to  supply  elemen- 
tary instruction  in  the  systematic  study  of  rocks. 
The  application  of  the  microscope,  in  this  special 
branch  of  geology,  has  of  late  years  afforded  more 
precise  information,  concerning  the  mineral  constitu- 
tion and  minute  structure  of  rocks,  than  it  was  pos- 
sible to  acquire  by  the  older  methods  of  research  ; 
and,  in  this  book,  I  have  endeavoured  to  give  a  clear 
explanation  of  the  method  of  preparing  sections  of 
rock  for  microscopic  examination,  as  well  as  a  des- 
cription of  the  microscopic  characters  of  the  most 
important  rock-forming  minerals,  upon  the  identifi- 
cation of  which  the  determination  of  the  precise 
character  of  a  rock  is  necessarily  based.  I  have 
been  compelled  to  make  very  free  use  of  foreign 


vi  Preface. 

works  on  petrology,  especially  those  of  Zirkel,  Rosen- 
busch,  and  Von  Lasaulx,  and  I  have  also  extracted 
much  information  from  other  foreign  and  British 
publications.  In  all  instances  I  have  endeavoured 
to  indicate  the  sources  from  which  the  information 
has  been  derived,  and  in  this  respect  I  trust  that 
no  injustice  has  been  done  to  any  author.  I  am 
also  greatly  indebted  to  Professor  A.  Renard,  of  the 
Royal  Museum  in  Brussels,  for  the  revision  of  one  or 
two  chapters  and  for  many  useful  hints.  I  would 
especially  thank  Professor  John  Morris  for  his  kind- 
ness in  voluntarily  undertaking  the  revision  of  the 
entire  work  ;  indeed  I  cannot  adequately  express  my 
obligation  to  him  for  his  friendly  and  valuable  cri- 
ticisms. 

Professor  Zirkel  has  kindly  given  me  permission 
to  copy  some  of  the  microscopic  drawings  from  his 
works,  and  has  also  assisted  me  by  sending  me  some 
of  his  publications. 

To  Professors  J.  W.  Judd,  A.  H.  Green,  and  T. 
G.  Bonney,  and  to  Messrs.  H.  W.  Bristovv,  J.  A. 
Phillips,  H.  Bauerman,  S.  Allport,  W.  Chandler 
Roberts,  Trenham  Reeks,  E.  Best,  T.  V.  Holmes, 
and  others,  I  am  also  indebted  for  information  or 
for  specimens  which  have  helped  me  in  the  prosecu- 
tion of  my  work,  while  from  the  Editor,  Mr.  C.  W. 
Merrifield,  I  have  received  a  multitude  of  useful 
suggestions. 

My  thanks  are  likewise  due  to  Mr.  E.  T.  Newton, 


Preface.  vii 

Assistant  Naturalist  to  the  Geological  Survey,  for 
some  interesting  notes  on  his  method  of  preparing 
microscopic  sections  of  coal,  and  to  Mr.  J.  B.  Jordan, 
of  the  Mining  Record  Office,  for  the  revision  of  the 
chapter  on  the  preparation  of  microscopic  sections, 
and  for  the  use  of  the  wood-block  representing  the 
construction  of  his  section-cutting  machine. 

In  the  classification  I  have  to  some  extent  deviated 
from  the  systems  commonly  adopted  ;  and,  in  the 
general  treatment  of  the  different  subjects,  original 
ideas  and  observations  are  more  or  less  plentifully 
interwoven  with  the  information  derived  from  books. 

From  the  limited  size  of  this  work  I  have  neces- 
sarily been  compelled  to  treat  certain  portions  of  the 
subject  with  brevity,  but  I  trust  that  nothing  of 
importance  to  the  student  has  been  omitted. 

I  have  intentionally  avoided  anything  more  than 
casual  references  to  localities  where  particular  rocks 
occur,  and  have  preferred  to  devote  additional  space 
to  the  descriptions  of  the  typical  rocks  themselves. 

If  I  appear  to  have  entered  too  much  into  micro- 
scopical details,  I  can  merely  observe  that  nature 
makes  no  difference  between  great  and  small  ;  that 
the  great  features  which  diversify  the  earth's  surface, 
and  which  Appear  stupendous  to  our  finite  perceptions, 
are  absurdly  trivial  when  compared  with  the  dimen- 
sions of  the  globe  itself,  while  the  latter,  in  relation 
to  the  sun,  is  a  mere  speck.  Minute  structure  and 
gross  are  alike  governed  in  their  development  by  the 


viii  Preface. 

same  natural  forces,  which  are  giants  commanding 
legions  of  atoms,  and  these  hosts  of  pigmies  constitute 
the  world.  If  the  present  power  of  assisting  vision 
were  amplified  thousands  of  times,  we  should  probably 
find  similarly  perfect  results,  governed  by  the  same 
laws,  the  general  principle  of  which  seems  remotely 
hidden  in  those  fields  of  inquiry,  which  fade  away 
on  every  side  into  the  regions  separating  human 
reason  from  Omniscience.  In  conclusion,  I  may  add 
that  petrology  cannot  be  learnt  merely  by  reading, 
and  that  this  little  work  does  not  pretend  to  be  more 
than  a  rudimentary  guide  to  the  subject. 

F.  R. 


CONTENTS. 

PART   I. 
THE  RUDIMENTS  OF  PETROLOGY. 

CHAPTER  I. 

METHOD   OF   RESEARCH,    ETC. 

PAGE 

Introductory  remarks  on  methods  of  petrological  research,  and  on 
the  scientific  and  practical  value  of  observations  on  the  chemical 
and  mineralogical  constitution  of  rocks I 

CHAPTER   II. 

ROCKS   DEFINED   AND   THEIR   ORIGIN   CONSIDERED. 

Rocks  are  mineral  aggregates — Conditions  of  aggregation — Preli- 
minary considerations  bearing  on  the  origin  of  rocks  .  .  6 

CHAPTER   III. 

DISTURBANCES   OF  THE   EARTH'S   CRUST — STRUCTURAL 
PLANES — SEDIMENTARY   ROCKS — STRATIGRAPHY. 

Subterranean  forces — Vulcanicity  and  Seismology — Evidence  of 
the  internal  heat  of  the  earth — Fissuring  and  displacement  of 
rock-masses  —  Structural  planes  —  Faults — Joints —  Laminar 
fission — Cleavage — Columnar  and  spheroidal  structure — Rocks 
divided  into  sedimentary  and  eruptive  groups — Stratification — 


Contents. 

PAGE 

Mode  of  formation  of  sedimentary  rocks— Clays,  sands,  lime- 
stones, &c.  —Classification  of  sedimentary  rocks — Dip  and 
strike  of  beds — Flexure  of  beds — The  influence  of  dip  and 
erosion  on  the  geological  map  of  a  country — Measurement  of 
the  thickness  of  beds — Denudation,  marine  and  atmospheric — 
Cliffs  and  escarpments — Weathering  of  rocks  dependent  on 
relative  hardness — Hills  and  valleys  —Erosion  of  limestones — 
Weathering  of  eruptive  rocks — Formations — Palseontological 
considerations — Unconformities  .  .  .  .  -9 


CHAPTER   IV. 

GENERAL  CHARACTER  AND  MODE  OF  OCCURRENCE  OF 
ERUPTIVE  ROCKS. 

Volcanic  and  plutonic  rocks — Basic  and  acid  rocks — Origin  of 
slaty  cleavage  —  Foliation  —  Metamorphism — Definition  of  a 
volcano — Volcanic  phenomena  .  .  .  .  ...  32 


CHAPTER  V. 

THE  COLLECTING  AND  ARRANGEMENT  OF   ROCK  SPECIMENS. 

On  the  collecting,  dressing,  labelling,  and  arrangement  of  rock 
specimens  ..........  39 

CHAPTER  VI. 

PRELIMINARY   EXAMINATION   OF   ROCKS. 
Hardness  tests — Description  of  needful  apparatus,  &c.  .         .     44 

CHAPTER  VII. 

THE   MICROSCOPE  AND   ITS  ACCESSORIES. 

Microscopes  suitable  for  petrological  work  —  Points  essential 
in  the  construction  of,  and  needful  apparatus  for  such 
microscopes — Goniometric  measurements — Stauro-microscope 
of  Prof.  Rosenbusch—  Inverted  microscope  .  .  .  .46 


Contents.  X1 


CHAPTER  VIII. 

METHOD  OF   PREPARING   SECTIONS  OF   MINERALS   AND 
ROCKS    FOR  MICROSCOPIC   EXAMINATION. 

PAGE 

Description  of  needful  materials  and  apparatus — Preparation  of 
chips  and  slices — Preliminary  grinding — Cementing  to  glass — 
Grinding  slab — Second  grinding  process— Final  grinding — Re- 
moval of  section  from  grinding  slab — Cleansing  and  mounting 
process — Preparation  of  sections  of  very  soft  rocks — Mr.  E.  T. 
Newton's  method  of  preparing  thin  sections  of  coal  .  .  59 


CHAPTER   IX. 

ON   THE   EXAMINATION   OF  THE  OPTICAL  CHARACTERS   OF 
THIN   SECTIONS   OF   MINERALS   UNDER  THE   MICROSCOPE. 

Phenomena  of  polarisation — Stauroscopic  phenomena  .         .         -74 


CHAPTER  X. 

THE   PRINCIPAL  ROCK-FORMING   MINERALS  :    THEIR   MEGA- 
SCOPIC AND   MICROSCOPIC   CHARACTERS. 

Species  of  the  felspar  group — Nepheline — Leucite — Scapolite  and 
Meionite— Sodalite,  Hauyne,  and  Nosean — Olivine — Hyper- 
sthene — Enstatite — Bronzite — Species  of  the  pyroxene  group — 
Species  of  the  amphibole  group— Species  of  the  mica  group — 
Chlorite  —  Talc — Tourmaline — Epidote  —  Sphene — Species  of 
the  garnet  group — Topaz — Zircon — Andalusite  and  Kyanite — 
Apatite — Rutile — Cassiterite — Calcspar — Quartz,  &c. — Magne- 
tite— Titaniferous  iron — Hematite — Limonite — Ironand  copper  • 
pyrites— Zeolites — Viridite— Opacite,  &c 86 


xii  Contents. 

PART   II. 
DESCRIPTIVE  PETROLOGY. 

CHAPTER  XI. 

THE  CLASSIFICATION  OF  ROCKS — ERUPTIVE  ROCKS, 
CLASS  I.  :  VITREOUS   ROCKS. 

PAGE 

Description  of  structures  developed  in  vitreous  rocks— Obsidian- 
Pumice— Perlite — Pitchstone — Tachylyte  .  .  .  .174 

CHAPTER  XII. 

ERUPTIVE  ROCKS,  CLASS  II.  :  CRYSTALLINE  ROCKS. 

Granitic  group — Syenite  group — Trachyte  group — Phonolite  group 
— Andesite  group — Porphyrite  group — Diorite  group  .  .  202 

CHAPTER   XIII. 

ERUPTIVE   ROCKS,    CLASS   II.  :   CRYSTALLINE   ROCKS    (cont.). 

Diabase  group — Gabbro  group — Basalt  group — Rocks  of  excep- 
tional mineral  constitution,  including  garnet  rock,  eklogite, 
Iherzolite,  &c. — Volcanic  ejectamenta — Altered  eruptive  rocks.  244 

CHAPTER   XIV. 

SEDIMENTARY  ROCKS. 

Unaltered  series :  Arenaceous,  argillaceous,  and  calcareous  groups 
— Altered  series,  including  porcelain-jasper  :  chiastolite-  and 
Staurolite -slates,  quartzite,  gneiss,  granulite ;  mica-,  chlorite-, 
talc-,  hornblende-,  and  other  crystalline  schists— Conglo- 
merates, breccias,  tufas,  and  sinters— Mineral  deposits  con- 
stituting rock-masses  ........  274 


APPENDIX .        .        .        .  307 

ERRATA 312 

INDEX       ...........  313 


Ll  HK;:-/-  ,;- 

:    NlVKi^lTV'oK 


STUDY  OF  ROCKS. 

PART    I. 
THE  RUDIMENTS  OF  PETROLOGY. 


CHAPTER   I. 

METHODS    OF    RESEARCH,    ETC. 

THE  means  at  the  disposal  of  the  older  petrologists  for  iden- 
tifying the  mineral  components  of  fine  grained  or  minutely 
crystalline  rocks  were  so  primitive,  that  we  wonder,  not 
so  much  at  the  little  that  was  known  about  them,  as  at 
the  quantity  of  information  amassed  by  such  simple  methods, 
and  at  the  truth  or  comparative  accuracy  of  many  of  their 
statements  bearing  directly  upon  this  subject.  The  pocket 
lens  was  one  of  their  most  important  implements  in  this 
work,  and  was  indeed  the  only  means  they  possessed  for 
distinguishing  minute  structure;  for  although  compound 
microscopes  were  known  and  used  for  physiological  pur- 
poses, still  the  idea  of  slicing  and  grinding  down  fragments 
of  rock  into  thin  sections  had  not  at  that  time  occurred  to 
anyone,  or,  if  it  had  done  so,  had  at  all  events  never  been 
carried  into  practice.  Chemical  analysis  and  simple  tests 
of  hardness,  specific  gravity,  &c,  such  as  have  been  given  in 
treatises  on  mineralogy  for  considerably  more  than  half  a 

B 


2  The -Riidiments  of  Petrology. 

century,  \vere  the  other, methods  which  they  were  enabled  to 
call  in  to  their  assistance  ;'  but. chemical  analysis  of  aggregates 
of  minute  and  undetermined  minerals  served  only  to  throw 
a  very  imperfect  light  upon  the  precise  nature  of  the  com- 
ponent minerals  themselves,  and  in  this  way  rocks  which 
differed  widely  in  minute  structure  and  in  mineral  composi- 
tion often  yielded  almost  identical  results  so  far  as  their 
ultimate  chemical  composition  was  concerned,  while  a  know- 
ledge of  the  physical  conditions  which  governed  them  at 
the  time  of  their  deposition  or  solidification  were  matters 
which  could  only  be  inferred  from  observations  made  in 
the  field,  upon  their  mode  of  occurrence  and  relation  to 
other  .rocks.  Vague  speculations  were  discussed  with  an 
energy  which  shows  how  deeply  these  pioneers  of  geology 
were  interested  in  this  branch  of  their  science,  but  from  the 
difficulties  which  attended  the  successful  prosecution  of 
these  inquiries,  especially  so  far  as  the  eruptive  rocks  were 
concerned,  the  study  of  petrology  as  a  special  geological 
subject  seemed  to  lapse  from  a  state  of  misplaced  energy 
into  one  of  hopeless  torpor.  The  first  steps  which  in  this 
country  tended  to  beget  fresh  ardour  in  this  direction  were 
the  publication  of  a  paper  by  H.  C.  Sorby,  in  the  '  Quarterly 
Journal  of  the  Geological  Society  of  London,'  vol.  xiv.  p.  453, 
4  On  the  Microscopical  Structure  of  Crystals,  indicating  the 
Origin  of  Minerals  and  Rocks,'  and  a  short  article  by  the 
late  David  Forbes,  in  the  '  Popular  Science  Review,'  Oct. 
1867,  entitled  'The  Microscope  in  Geology.'  The  obser- 
vations recorded  in  these  papers  were  based  upon  the 
microscopic  examination  of  thin  sections  of  minerals  and 
rocks  ;  and  although  Mr.  Sorby  appears  to  have  been  the 
first  to  apply  this  kind  of  examination  to  purely  mineralogical 
and  petrological  questions,  still  the  method  of  grinding  such 
thin  sections  for  microscopic  work  was  first  practised  by  H. 
Witham  in  1831,  when  conducting  researches  on  the  minute 
internal  structure  of  fossil  plants.  The  great  advantage 
derived  from  the  examination  of  thin  sections  of  minerals 


Microscopic  Examination  of  Rocks.  3 

lies  in  the  circumstance,  that  in  many  cases  a  mineral  which 
in  ordinary  hand-specimens,  in  thick  splinters  or  in  thick 
slices,  would  appear  to  be  opaque,  is  rendered  more  or  less 
translucent  or  transparent,  the  transparency  increasing  with 
the  thinness  of  the  section,  so  that  structures  which  in  many 
instances  could  not  be  discerned  by  reflected  light  are 
rendered  apparent  when  the  specimen  is  thin  enough  to  be 
moderately  translucent,  while,  in  conjunction  with  the  micro- 
scope, the  polariscope,  spectroscope,  and  goniometer  may 
be  used,  and  additional  facilities  are  thus  given  for  examin- 
ing the  optical  properties  of  the  mineral.  Such  advantages 
accrue  from  the  preparation  of  thin  slices  of  rocks,  the  com- 
ponent minerals  of  which  may  be  studied  microscopically, 
their  crystallographic,  optical,  and  other  physical  properties 
noted,  even  in  rocks  whose  texture  is  so  fine  that  examina- 
tion with  ordinary  hand  lenses  is  insufficient  to  give  any 
clear  insight  as  to  the  .nature  of  their  components,  while 
opportunities  are  thus  offered  for  acquiring  much  information 
about  the  paragenesis  of  minerals,  the  physical  conditions 
under  which  rocks  have  been  formed,  and  the  changes  both 
physical  and  chemical  which  they  have  subsequently  under- 
gone. It  is  therefore  manifest  that  the  preparation  of  thin 
slices  of  minerals  and  rocks  has  led  the  geologists  of  the 
present  day  into  a  vast  and  hitherto  unexplored  field  of 
inquiry,  in  which  new  questions  will  propound  themselves, 
and  old  ones,  in  time,  find  their  solution. 

The  benefits  to  science  which  are  likely  to  accrue  from  a 
steady  prosecution  of  such  studies  are,  from  the  foregoing 
remarks,  sufficiently  obvious,  but  many  people  may  be 
prone  to  think  that  little  practical  advantage  is  likely  to  be 
derived  from  this  branch  of  microscopic  research.  Such 
objectors,  if  asked  whether  the  texture  and  quality  of  build- 
ing stones,  bricks,  and  mortars  were  matters  of  conse- 
quence in  architectural  work,  would  probably  reply  in  the 
affirmative,  and  would  then  most  likely  add  :  *  Such  matters 
can  be  decided  by  experience,  by  noticing  buildings  both 

B  2 


4  The  Rudiments  of  Petrology. 

old  and  new,  and  seeing  how  the  different  materials  of  which 
they  are  built  have  resisted  the  ravages  of  time  and  expo- 
sure, and  if  we  want  further  information  we  can  have  the 
materials  analysed  by  a  chemist,  and  he  will  be  able  to  tell 
us  all  we  need  to  know.'  Such  a  remark  as  this  would 
embody  a  great  deal  of  truth.  The  chemist  could  furnish 
an  analysis  of  the  stone  itself,  and  we  should  thus  learn  how 
much  lime,  or  magnesia,  or  silica,  or  carbonic  acid,  &c.,  &c., 
was  contained  in  it;  but  if  the  stone  happened  to  be  a  very 
fine-grained  one,  and  although  in  freshly  quarried  samples 
apparently  homogeneous,  yet  when  exposed  to  the  weather 
it  suffered  unequal  decomposition  or  disintegration,  it  would 
be  clear  that  atmospheric  agencies  detected  weak  spots 
better  than  the  chemist,  and  better  than  the  practised  eyes 
of  geologists,  architects,  stonemasons,  and  quarrymen.  It 
would  then  be  evident  that  more  practically  useful  informa- 
tion could  be  derived  from  inspection  of  buildings  than  from 
the  sources  just  enumerated;  but  at  last  the  awkward  ques- 
tion arises,  *  How  is  it  that  stones  which  often  have  the 
same,  or  almost  the  same,  chemical  composition,  and  which 
also  closely  resemble  one  another  in  appearance,  have  dif- 
ferent powers  of  resisting  the  effects  which  result  from 
exposure  ? '  It  is  true  that  the  stones  most  affected  may  not 
have  been  judiciously  laid;  it  is  true  that  the  atmosphere  of 
large  towns  is  more  prejudicial  to  building  stones  than  the 
purer  atmosphere  in  country  places ;  but,  given  similar 
conditions,  why  is  it  that  apparently  similar  stones  wear 
differently  ?  Those  who  have  examined  the  minute  struc- 
ture and  mineral  composition  of  rocks  well  know  how 
little  dependence  is  to  be  placed  on  outward  similarity, 
and  even  in  chemical  analysis ;  for  changes  go  on  within 
rocks  which  often  produce  little  or  no  definite  chemical 
change  so  far  as  the  aggregate  is  concerned,  but  beget 
numbers  of  little  interchanges  in  the  chemical  composition 
and  molecular  arrangement  of  the  component  minerals. 
Although  in  the  present  state  of  our  knowledge  but  little 


Practical  Applications  of  Petrology.  5 

practical  use  has  yet  been  made  of  the  recorded  observa- 
tions which  now  constitute  merely  the  small  nucleus  of  what 
will  no  doubt  eventually  become  a  huge  pile  of  information, 
still  we  may  look  forward  to  the  time  when  a  knowledge  of 
the  minute  structure  of  rocks  will  be  recognised  as  indis- 
pensable to  the  right  understanding  of  the  changes  which 
building  stones  undergo,  and  when  not  merely  the  few  but 
the  many  will  be  benefited  by  this  branch  of  scientific 
inquiry.  A  general  knowledge  of  petrology  will  always  be 
found  useful  by  those  who  may  have  to  deal  with  architec- 
ture or  with  mining  enterprises,  and  it  is  to  be  hoped  that 
some  day,  as  science  progresses,  a  definite  connection  may 
be  found  to  exist  between  metalliferous  lodes  and  the  min- 
eral composition  of  the  rocks  in  which  such  lodes  occur. 
Questions  of  water  supply  hinge  mainly  upon  the  porous  or 
impervious  character  of  rocks,  upon  their  mode  of  occur- 
rence, and  upon  the  structural  planes  or  planes  of  dislocation 
by  which  they  are  traversed,  so  that  matters  of  this  kind  can 
be  best  dealt  with  by  the  field  geologist.  In  concluding 
this  brief  introduction,  it  seems  needful  to  caution  the  student 
not  to  regard  petrology  from  a  narrow  point  of  view ;  not 
to  confine  his  attention'  solely  to  observations  in  the  field; 
nor  to  devote  himself  exclusively  to  microscopical  or  che- 
mical research^  The  disadvantage  under  which  the  spe- 
cialist labours  is,  that  he  frequently  takes  infinite  trouble  to 
unravel  a  question  in  his  own  special  way,  when  by 
adopting  some  other  method  he  might  arrive  at  his  result 
in  far  less  time,  and  often  with  greater  certainty.  At  times 
a  penknife  will  be  found  more  useful  than  a  blowpipe,  and  a 
blowpipe  than  a  microscope  ;  at  other  times  a  microscope 
will  tell  more  than  a  complete  chemical  analysis. 


6  The  Rudiments  of  Petrology. 

CHAPTER   II. 

ROCKS    DEFINED   AND   THEIR   ORIGIN    CONSIDERED. 

IF  we  examine  a  fragment  of  rock,  we  find  it  to  consist  as  a 
rule  of  crystals,  the  edges  and  angles  of  which  may  either  be 
sharply  denned  or  rounded,  and  which  are  cemented  together 
either  by  crystalline  or  amorphous  mineral  matter,  or  we  may 
find  it  composed  of  large  or  very  minute  angular  fragments 
of  mineral  matter,  or  of  rounded  grains,  or  of  a  mixture  of 
both  angular  and  more  or  less  rounded  grains  also  bound 
together  by  mineral  matter,  which  may  either  be  amorphous 
or  may  possess  a  crystalline  structure.  Sometimes,  however, 
the  grains  simply  cohere  without  any  perceptible  cement,  as 
in  some  of  the  new  red  and  other  sandstones.  These  kinds  of 
rocks  may  be  denned  as  mineral  aggregates,  but  the  term 
1  rock '  in  its  geological  signification  does  not  merely  imply 
a  coherent  mass  but  also  loose  incoherent  mineral  matter, 
such  as  blown  sand,  and  in  these  cases,  as  in  the  preceding 
ones,  the  materials,  whether  they  consist  of  fragments  of  one 
mineral  only  or  of  several  different  minerals,  may  still  be 
regarded  as  mineral  aggregates.  There  are,  however,  ap- 
parent exceptions,  for  some  rocks  appear  to  the  naked  eye 
to  be  perfectly  homogeneous.  Some  quartzites  may  be  so 
regarded,  but  we  know  that  passages  have  been  observed 
between  quartzites  and  fine  grained  sandstones.  A  casual 
observer  might  also  take  such  a  rock  as  Lydian-stone  or 
Hone-stone  to  be  quite  homogeneous,  but  examination  of  a 
thin  slice  under  the  microscope  would  show  it  to  consist  of 
numerous  lenticular  particles,  which,  from  their  form  alone, 
imply  the  necessity  for  and  the  consequent  existence  of  a 
cementing  matter  which  differs  from  the  particles  themselves. 
Again  obsidians,  pitchstones,  and  other  vitreous  rocks,  would 
be  assumed  by  the  general  observer  to  be  perfectly  homo- 
geneous, but  here  again  the  microscope  demonstrates  that 
they  contain  fine  dusty  matter,  microliths,  and  crystals  in 


The  Origin  of  Rocks.  7 

great  quantity ;  so  that  even  those  rocks  which  are  appa- 
rently exceptions  to  the  general  definition  are  found  in 
reality  to  conform  to  it,  and  thus  we  may  with  considerable 
truth  define  all  rocks  as  mineral  aggregates. 

The  state  of  aggregation  of  a  rock  depends  upon  the 
way  in  which  it  was  formed  and  the  changes  which  it  may 
subsequently  have  undergone.  We  do  not  know  what  was 
the  character  of  the  rock  or  rocks  which  formed  the  crust 
when  the  surface  of  the  heated  mass  which  originally  con- 
stituted the  globe  first  solidified.  The  older  geologists  were 
Lnd  of  speculating  upon  this  subject,  and  many  of  them 
believed  that  the  primeval  crust  was  granite,  and  they  further- 
more believed  that  all  granite  could  claim  this  high  antiquity. 
These  were,  however,  mere  speculations,  and  granites  are 
now  known  to  be  of  various  ages,  some  of  them  having  been 
formed  in  comparatively  late  geological  times.  Great  mis- 
conception seems  to  exist  among  geologists  even  at  the 
present  day  about  the  origin  of  rocks,  and  these  misconcep- 
tions usually  resolve  themselves  into  bickering  about  terms 
and  the  way  in  which  they  should  be  employed.  It  therefore 
seems  desirable,  at  the  very  outset,  to  lay  before  the  student 
a  few  preliminary  considerations  which  may  help  him  to 
think  and  work  in  a  systematic  manner. 

1 i )  Assuming  that  the  earth  was  originally  a  molten  mass 
revolving  in  space,  and  that  after  considerable  radiation  had 
taken  place  solidification  of  the  surface  ensued,  it  is  clear 
that  that  primitive  crust  was  the  first  rock  formed. 

(2)  Assuming  that  radiation  has  constantly  been  going 
on  from  the  earliest  times  up  to  the  present  day,  it  is  evident 
that  the  whole  mass  of  rock  which  now  constitutes  the  crust 
of  the  earth  represents  the  entire  work  of  solidification,  no 
matter  what  may  be  the  character  (whether  sedimentary,  erup- 
tive, 6°^.)  of  those  rocks  now,  unless,  through  changes  in  the 
interior  of  the  globe,  chemical  action  has  been  called  into 
play,  and  has  thereby  generated  additional  heat,  neutralising 
to  some  extent  the  work  of  solidification  by  fusing  again  the 
rocks  which  came  in  contact  with  these  highly  heated  masses, 


8  The  Rudiments  of  Petrology. 

or,  unless  through  fracture  and  disturbance  of  the  already 
solidified  crust  (by  the  expansive  force  of  gases  generated 
by  chemical  changes,  or  by  physical  changes,  such  as  the 
conversion  of  water  into  steam),  portions  of  the  already  so- 
lidified crust  were  faulted  down  or  depressed  so  as  to  come 
within  the  range  of  these  heated  portions  of  the  earth,  in 
which  case,  as  in  the  preceding,  some  of  the  pre-existing  work 
of  solidification,  from  loss  of  heat,  by  radiation,  would  be 
undone,  and  the  total  mass  of  solid  rock  now  constituting  the 
crust  would  then  represent  the  total  amount  of  rock  formed 
by  solidification  through  radiation,  minus  the  amount  of 
rock  re-fused. 

(3)  The  rocks  now  called  plutonic,  and  which  have 
solidified  at  various  depths  beneath  what  was  the  surface  of 
the  earth  at  the  time  of  their  solidification,  may  be  regarded 
as  the  result  of  the  solidification  of  the  earth's  magma  through 
loss  of  heat  by  radiation  •  the  rocks  now  called  volcanic,  and 
which  have  been  erupted  through  the  crust  and  solidified  on 
what  was  once,  or  on  what  is  now,  the  surface  of  the  earth 
(whether  subaerial  or  marine),  may  be  regarded  merely  as 
outwardly-trending  phases  of  the  plutonic  rocks,  and  are 
therefore  also  the  products  of  solidification  through  loss  of 
heat  by  radiation  ;  the  rocks  now  called  aqueous  or  sedi- 
mentary are  the  result  of  the  degradation  of  land  by  sub- 
aerial  or  marine  denudation,  and  because  the  first  land  must 
of  necessity  have  been  portions  of  the  crust  formed  by  the 
solidification  of  the  earth's  surface  through  radiation.  Lastly, 
the  rocks  now  called  metamorphic  are  merely  either  eruptive 
(i.e.  plutonic  and  volcanic  rocks)  or  else  sedimentary  rocks, 
which  have  subsequently  undergone  changes  either  physical 
or  chemical,  and  are  therefore  only  the  products  of  the  so- 
lidification of  the  earth's  magma  through  radiation,  in  an 
altered  condition.  Consequently  no  sedimentary  rock  has 
ever  been  formed  except  from  materials  which,  in  the  first 
instance,  were  supplied  in  a  solid  form  through  the  radiation 
of  heat  from  the  globe. 


Vtdcanicity  and  Seismology. 


CHAPTER  III. 

DISTURBANCES    OF   THE    EARTH'S    CRUST. — STRUCTURAL 
PLANES. — SEDIMENTARY    ROCKS. — STRATIGRAPHY. 

THE  study  of  forces  existing  in  the  interior  of  the  earth,  and 
the  phenomena  attendant  upon  their  exertion  and  affecting 
the  earth's  crust,  constitute  the  branches  of  physical  geology 
known  as  vulcanicity  and  seismology,  the  former  relating  to 
volcanic  phenomena  and  the  latter  to  earthquakes.  These 
forces  are  doubtless  due  to  chemical  and  physical  reactions 
and  changes,  resulting  in  the  development  of  intense  heat 
and  the  generation  of  gases.  It  is  also  assumed,  however,  that 
this  high  subterranean  temperature  is  mainly  owing  to  the 
original  heat  of  the  globe  when  first  developed  as  an  indivi- 
dual molten  mass,  only  part  of  this  heat  having  been  dissi- 
pated by  radiation  into  space.  The  consequent  loss  of  heat 
having  taken  place  from  the  exterior  of  the  globe,  and  most 
affecting  the  superficial  portion  of  it,  has  given  rise  to  the 
formation  of  its  crust,  by  solidification  of  the  once  molten 
matter.  It  seems  reasonable  to  suppose  that  such  radiation 
took  place  equally  over  the  entire  surface,  and  that  the 
phenomena  consequent  upon  the  cooling  of  the  mass, 
although  doubtless  affecting  the  centre  of  the  globe,  diminish 
in  their  intensity  from  the  surface  inwards.  These  physical 
changes  probably  extend  inwards  towards  the  centre  with  a 
certain  amount  of  regularity,  so  that  the  globe  might  pos- 
sibly be  regarded  as  a  spheroidal  mass  consisting  of  a  series 
of  zones  varying  in  temperature  and  augmenting  as  the 
central  portions  are  reached.  Assuming  this  to  be  the  case, 
there  would  be  a  zone  situated  at  some  depth  beneath  the 
surface,  whose  temperature  would  be  so  high  that  any  known 
rock  matter  could  no  longer  exist  in  a  solid  state.  The 
depth  at  which  this  zone  of  fusion  is  supposed  to  occur  has 
been  variously  estimated  by  different  writers.  From  twenty- 


IO  TJie  Rudiments  of  Petrology. 

five  to  thirty  miles  is  about  the  smallest  estimate  which  has 
been  given  ;  while  some,  as  Hopkins,  have  inferred  that  solid 
matter  extends  to  much  greater  depths  and  may  even  exist 
at  the  centre,  the  loss  of  heat  and  consequent  solidification 
having  taken  place  in  an  irregular  manner,  and  having  thus 
converted  the  deeper  portions  of  the  globe  into  a  somewhat 
honey-combed  mass,  the  cavities  still  retaining  matter  in  a 
molten  condition,  and  constituting  the  reservoirs  from  which 
the  eruptive  rocks  are  derived.  So  many  theories  upon  this 
subject  have  from  time  to  time  been  started,  and  they 
embody  such  diversity  of  opinion,  that  a  description  of  them 
would  be  out  of  place  in  so  small  a  book ;  and  as  they  cannot 
be  regarded  as  more  than  speculations,  often  based  upon  a 
little  tangible  truth  and  more  or  less  tangible  and  intangible 
error,  students,  although  doing  well  to  make  themselves 
acquainted  to  some  extent  with  these  theories,  would  do 
better  in  giving  their  attention  to  matters  which  are  more 
readily  demonstrable. 

That  the  forces  just  spoken  of  as  existing  in  the 
interior  of  the  earth  exercise  considerable  influence  upon 
its  crust  we  have  ample  evidence.  It  is,  indeed,  solely 
from  such  evidence  that  we  infer  the  existence  of  the 
forces. 

The  evidence  which  we  have  of  the  internal  heat  of 
the  earth  may  be  summed  up  under  the  following 
heads  : — 

(1)  In  descending  the  shafts  of  mines  a  gradual  rise  of 
the  thermometer  takes  place  after  the  descent  of  the  first 
sixty  feet.     Down  to  this  point  it  remains  stationary  ;  below 
this  point  there  is  a  rise  of  one  degree  Fahrenheit  for  every 
sixty   feet   descended.     How   far   this   regular   increase   of 
temperature  continues    to    take    place    has    not    yet   been 
determined. 

(2)  Flows  of  molten  lava  and  of  hot  mud,  the  ejection 
of  lapilli  and  ashes  from  volcanic  vents  through  the  genera- 
tion of  steam  or  the  evolution  of  gases,  and  the  occurrence 


Dislocations  of  tJie  EartJis  Crust.  1 1 

of  geysers  and  thermal  springs,  are  also  evidences  of  the 
internal  heat  of  the  earth. 

(3)  Earthquakes,  the  elevation  and  depression  of  large 
tracts  of  land,  giving  rise  to  changes  in  coast  lines,  as  in 
Greenland,  Sweden,  and  South  America,  at  the  present  day; 
the  fractures  produced  in  the  crust  by  subterranean  forces, 
and  the  relative  displacement  of  rock  masses  along  such 
lines  (faulting)  ;  the  bending  and  contortion  which  stratified 
rocks  undergo,  and  the  chemical  and  physical  changes  (meta- 
morphism)  which  they  sometimes  experience,  are  all  evi- 
dences of  the  internal  heat  of  the  earth. 

That  the  fissuring  and  displacement  of  portions  of  the 
earth's  crust  is  frequently  due  to  the  exercise  of  the  subter- 
ranean forces  just  mentioned  there  is  not  the  least  doubt, 
since  they  have  occurred  frequently,  not  merely  within  his- 
torical times,  but  men  now  living  have  been  eye-witnesses  of 
many  remarkable  changes  which  have  been  brought  about 
in  the  structure  of  districts  by  seismic  action,  as  in  Calabria 
and  in  the  country  at  the  mouth  of  the  Indus.1  Moreover, 
many  of  the  disturbances  of  the  earth's  crust  which  have 
happened  in  even  very  remote  geological  periods  are  pre- 
cisely such  as  we  should  attribute  to  the  same  forces  which 
have  produced,  and  are  still  producing,  the  modern  dis- 
turbances. We  may  conveniently  classify  them  all  under 
certain  heads,  so  far  as  they  relate  to  structural  planes  ; 
either  on  a  large  'scale,  as  affecting  the  general  geology  of  a 
country,  or  on  a  smaller  scale,  as  influencing  not  merely  the 
gross  but  the  minute  and  even  microscopic  structure  of  rock 
masses.  It  may  here  be  remarked  that  the  scenery  and 
general  configuration  of  a  district  is  often  due  rather  to  the 
facilities  offered  for  the  weathering  of  rocks  along  small  and 
closely  disposed  planes  of  fission  than  to  the  presence  of 
long  lines  of  fracture  and  faulting.  The  latter  tend  to  produce 

1  For  descriptions  of  these  and  other  kindred  phenomena  the 
student  should  consult  Lyell'o  Principles  of  Geology,  in  which  accounts 
are  given  of  the  most  important  earthquakes  and  volcanic  eruptions 
which  have  occurred  within  historical  times. 


12  The  Rudiments  of  Petrology. 

lithological  diversity  of  surface  rather  than  diversity  of  con- 
tour or  relief;  atmospheric  agencies  apparently  producing 
little  or  no  effect  upon  one  colossal  divisional  plane,  since  a 
fault  seldom  developes  a  feature  in  any  landscape,  other 
than  perhaps  a  difference  in  its  vegetation.  Atmospheric 
degradation,  however,  along  innumerable  divisional  planes 
of  very  trivial  dimensions  gives  rise  to  outlines  which 
often  enable  a  practised  observer  to  discriminate  between 
different  formations  merely  from  the  aspect  which  they  have 
derived  from  weathering. 

The  following  table  gives  a  rough  and  rudimentary  classi- 
fication of  these  structural  planes  : — 

CLASSIFICATION  OF  STRUCTURAL  PLANES. 

1.  Irregular      ('  Earthquakes  and  their  attendant  phenomena, 
fissures       \      producing  fracture  of  the  crust  by  pressure 
and  faults.  (      from  within  directed  outwards. 

"  Shrinkage  on  consolidation  of  sediment  by  dry- 
ing and  consequent  contraction,  producing 

fracture  of  the  crust  usually  along  more  or 

2.  Jointing  „  ,  ,. 

,          '       H      less  parallel  lines. 

Shrinkage  on  consolidation  of  eruptive  matter  by 
cooling  and  consequent  contraction,  producing 
fracture  in  directions  more  or  less  parallel. 

C  Pressure,  exerted  by  contiguous  rock  masses, 
producing  (often  by  re-arrangement  of  par- 
ticles) planes  of  weak  cohesion,  along  which 


Laminar 


fission  readily  takes  place  in  parallel  direc- 


fissionand-^       tions. 

cleavage.        A.  Coincident  with    bedding  planes     (laminar 
fission  or  flaggy  cleavage). 

I  B.  Deviating  from  the  direction  of  the  bedding 

I.      planes  (slaty  cleavage). 

It  is  possible  that  many  of  the  lines  along  which  faulting 
has  taken  place  may  in  the  first  instance  have  been  simply 
fissures  due  to  shrinkage  (Class  2)  but  in  other  cases  faulting 


Origin  of  Faults.  1 3 

has  occurred  along  planes  produced  by  seismic  action 
(Class  i). 

The  parallelism  which  so  often  characterises  a  system  of 
faults  cannot,  however,  be  adduced  as  proof  that  those  faults 
have  occurred  along  joint  planes  (Class  2),  since  parallel  fis- 
sures might  be  produced  by  the  upheaval  of  rocks  along 
a  certain  line  or  rather  along  a  definitely  trending  area 
(Class  i)  on  either  side  of  which  relative  displacement  of 
strata  or  of  eruptive  rock  masses  may  have  subsequently 
occurred. 

It  is  also  quite  possible  that  the  displacement  itself 
originated  synchronously  with  the  line  or  plane  along  which 
it  runs,  in  which  case  the  plane  would  again  belong  to  Class  i. 
All  that  we  can  therefore  safely  say  about  the  origin  of 
faults  is,  that  they  are  relative  displacements  of  the  earth's 
crust,  caused  by  subterranean  forces  upheaving  masses  of 
rock  along  lines  of  least  resistance,  which  may  either  be  pro- 
duced at  the  time  of  upheaval  or  may  have  pre-existed 
simply  as  fissures  or  cracks,  and  that  in  some  cases  depres- 
sion of  rock  masses  has  caused  faults,  the  subsidence  of  the 
downthrow  having  occurred  by  the  mere  gravitation  of  the 
mass  between  two  outwardly  diverging  planes  of  fracture,  as 
in  the  case  of  '  trough  faults.' l 

Faults  may  also  arise  from  an  unequal  horizontal  shifting 
of  undulating  beds  along  a  fissure,  or  from  partial  flexure  or 
bagging  down  of  strata  upon  one  side  only  of  a  fracture, 
the  beds  on  the  other  side  remaining  horizontal.2 

The  fissures  and  cracks,  therefore,  along  which  faulting 
has  taken  place,  may  be  due  either  to  volcanic  or  seismic 
action  (for  earthquakes  and  volcanic  phenomena  are  so  inti- 
mately related,  and  are  apparently  so  indisputably  due  to 
similar  or  identical  causes,  that  they  may  safely  be  classed 
together),  or  to  the  shrinkage  of  sedimentary  rocks  by  loss  of 

1  Vide  Students  Mamtal  of  Geology,  by  J.  Beete  Jukes  (1862), 
p.  260. 

-  Ibid.  pp.  254-5. 


14  The  Rudiments  of  Petrology. 

moisture  and  that  of  eruptive  rocks  by  loss  of  heat  during 
solidification. 

Another  class  of  small  structural  cracks  which  occur  in 
some  eruptive  rocks  and  also  exceptionally  in  argillaceous 
beds  which  have  undergone  considerable  desiccation,  is,  in 
the  opinion  of  many  observers,  due,  in  the  case  of  the  erup- 
tive rocks,  such  as  basalt,  to  contraction  on  cooling.  These 
planes  intersect  one  another  in  such  a  manner  as  to  divide 
the  mass  of  rock  into  a  series  of  closely  packed  prisms 
varying  at  times  in  the  number  of  their  sides  and  in  the 
measurement  of  their  angles.  This  structure  is  especially 
characteristic  of  the  basalts  in  some  districts  (Staffa,  Giant's 
Causeway,  Unkel  on  the  Rhine,  parts  of  Auvergne,  and 
many  other  localities.)  The  prisms  are  generally  cut  trans- 
versely by  numerous  divisions,  which  are  sometimes  flat, 
sometimes  either  convex  or  concave,  while  occasionally,  as 
in  the  celebrated  Kasekeller,  they  consist  of  superposed 
spheroidal  lumps  or  balls,  which  have  a  concentric  shaly 
structure.  An  analogous  structure  on  a  very  small  and 
often  purely  microscopic  scale  is  to  be  met  with  in  vitreous 
rocks  such  as  perlite.1  According  to  the  theory  held  by  Sir 
Henry  De  la  Beche  and  others,  mountain  chains  owe  their 
origin  in  many  cases  to  immense  fractures  and  dislocations 
of  the  earth's  crust  caused  by  unequal  contraction  of  the 
crust  in  zones,  the  inner  zones  contracting  and  leaving  the 
outer  and  already  solidified  zone  unsupported,  so  that  in 
places  it  cracked,  large  masses  subsided  on  to  the  lower 
zone,  and  thus  caused  immense  ridges  and  depressions. 
Such  mountain -forming  fissures,  colossal  though  they  may 
be,  are,  however,  hypothetical  rather  than  demonstrable. 

Enough  has  now  been  said  to  show  that  structural  planes 
and  divisions  occur  in  rocks  ranging  from  those  of  gigantic 
size  to  others  of  quite  microscopic  dimensions.  Some  of 
them  occur  in  rocks  of  eruptive  origin,  some  traverse  sedi- 

1  Bonney,  Q.  J.  G.  S.,  vol.  xxxii.  p.  140  ;  Allport,  Q.  J.  G.  S., 
vol.  xxxiii.  p.  449  ;  Rutley,  Trans.  7u  Mic.  Soc.,  vol.  xv.  p.  176. 


Sedimentary  and  Eruptive  Rocks.  1 5 

mentary  deposits,  but  in  all  cases  they  facilitate  the  weather- 
ing and  disintegration  of  the  rocks  in  which  they  occur,  and 
consequently  exercise  a  more  or  less  marked  effect  upon  the 
scenery  of  a  district. 

The  rocks  composing  the  crust  of  the  earth  may  be  con- 
sidered mainly  to  belong  to  two  great  divisions,  viz.,  (A) 
the  aqueous,  sedimentary,  fossiliferous,  or  stratified  rocks, 
which  have  been  deposited  as  sediment  in  beds,  or  strata 
beneath  water,  each  bed  or  stratum  having  successively 
formed  the  floor  of  a  sea,  or  of  a  lake ;  and  (B)  igneous 
or  eruptive  rocks,  which  have  formed  intrusive  bosses,  or- 
dykes,  or  have  been  poured  out  from  volcanic  vents,  as 
lava  flows. 

The  former  usually  contain  organic  remains  which  may 
be  identified  with  a  marine  or  a  lacustrine  fauna,  and  con- 
sequently afford  a  tolerably  safe  clue  to  the  circumstances 
under  which  the  beds  were  deposited.  It  may  be  safely 
assumed  that  all  such  beds  were  originally  spread  out  in  an 
approximately  horizontal  position,  and  that  any  strong 
deviation  from  the  horizontal  position  which  may  be  shown 
by  planes  of  bedding  is  due  to  subsequent  disturbance  of 
those  beds. 

Sediments  may  sometimes,  however,  be  somewhat  irre- 
gularly deposited;  for  example,  a  number  of  thin  beds  may 
thin  out  completely,  overlie  one  another,  and  the  whole  of 
them  may  overlie  a  perfectly  horizontal  bed  upon  which  their 
thinned-out  ends  appear  to  rise  unconformably,  and  this  kind 
of  arrangement  may  be  repeated  again  and  again  through  a 
considerable  thickness  of  deposits.  This  irregular  kind  of 
stratification  is  called  '  false  bedding.  '  Fig.  i  represents 
a  good  example,  occurring  in  the  lower  greensand,  at  Frith 
Hill,  near  Godalming. 

In  such  a  case  the  inclination  of  the  beds  is  not  due 
to  any  disturbances  during  or  subsequent  to  deposition, 
but  simply  to  the  overlap  of  successive  deposits  as  they 
are  thrown  down  in  shallow  water.  The  sediments  which 


i6 


The  Rudiments  of  Petrology. 


constitute  stratified  rocks  result  from  the  wear  and  tear 
which  takes  place  from  the  action  of  rain  on  land  sur- 
faces, and,  in  the  beds  of  rivers  from  attrition,  each  eroded 
fragment  serving  as  a  tool  with  which  other  fragments 
are  ground  away  from  the  rock.  When  a  turbid  river 
empties  itself  into  the  sea  or  into  a  lake,  the  materials  held 
in  suspension  become  deposited  according  to  their  relative 
specific  gravities,  the  heavier  fragments  sinking  first,  while 
the  lighter  particles  are  carried  to  a  greater  distance  from  the 

FIG.  i. 


river's  mouth.  The  relative  sizes  and  shapes  of  the  frag- 
ments also  exercise  some  influence  on  the  sorting  process 
which  takes  place.  Fragments  which  have  undergone  but 
little  attrition  are  usually  more  or  less  angular  in  form,  while 
those  which  have  been  carried  long  distances,  and  which 
have  been  rubbed  together  for  a  length  of  time,  become  sub- 
angular  or  perfectly  rounded.1  The  rounded  form  of  the 
pebbles  on  sea-beaches  is  due  to  the  incessant  grinding 

1  Except  in  instances  where  the  fragments  have  been  transported 
by  ice. 


Stratified  Rocks.  1 7 

which  they  undergo  against  one  another  during  the  advance 
and  retreat  of  every  wave  that  washes  the  shore. 

Deposits  mainly  composed  of  angular  fragments  are  termed 
breccias,  while  those  consisting  of  rounded  pebbles  are  called 
conglomerates.  In  indurated  rocks  of  this  kind  the  coarse  frag- 
ments, or  pebbles,  are  generally  cemented  together  by  a  finer 
material,  often  consisting  of  carbonate  of  lime  or  silica.  There 
are  frequently  other  substances,  however,  which  act  as  a  cement. 

Aqueous  or  sedimentary  rocks  may  be  conveniently 
classed  as  follows  : — 

(1)  Clays,  which  when  indurated  become  mudstones,  and, 
when  cleaved,  slates^     When  they  merely  exhibit  a  fissile 
character  in  the  direction  of  the  lamination,  or  bedding,  they 
are  called  shales.    Clays,  slates,  and  shales  are  mainly  com- 
posed of  hydrous  silicate  of  alumina.     There  are  arenaceous 
and  calcareous  clays,  slates,  and  shales  ;  calcareous  clays  are 
termed  marls. 

(2)  Sands. — These  when  indurated  constitute  sandstones, 
and  when  more  or  less  coarse-grained,  and  composed    of 
angular  or  sub-angular  grains  of  sand  (frequently  with   an 
admixture  of  fragments  of  other  minerals),  they  are  then 
termed  grits.    The  term  '  grit '  is,  however,  very  loosely  used, 
and  it  would  be  difficult  to  give  it  a  sharp  definition  owing 
to  the  great  variation  in  the  physical  and  mineralogical  cha- 
racters of  the  rocks  to  which  this  name  has  been  applied. 
Sands  and  sandstones  are  usually  composed  of  fine  grains 
of  quartz  cemented  either  by  carbonate  of  lime,  carbonate 
of  iron,  oxides  of  iron,  or  silica.     There  are  calcareous  and 
argillaceous  sandstones. 

(3)  Limestones. — These  may  vary  from  soft  and  earthy, 
to  hard,  compact,  and  even  finely  crystalline  rocks.     Some 
limestones  may  be  merely  eroded  granules  of  pre-existing 
limestone  carried  mechanically  in  suspension  in  water,  and 
ultimately  deposited  as  a  sediment.     Some  may  have  re- 
sulted from  the   precipitation  of  carbonate  of  lime  from 
water  holding  the  bicarbonate  of  lime  in  solution.     In  this 

c 


1 8  The  Rudiments  of  Petrology. 

case  the  deposit  may  be  considered  to  have  a  chemical  origin. 
The  cause  of  precipitation  would  be  the  elimination  of  one  atom 
of  carbonic  anhydride  from  each  molecule  of  bicarbonate  of 
lime.     Travertine,  calcareous  tufa,    and   pisolite  are  rocks 
formed  in  this  manner.     The  last  consists  of  rounded  grains 
like  shot  or  peas,  whence  the  name  pisolite  or  peastone ;  these 
little  pellets  consist  of  a  series  of  concentric  coats  of  carbonate 
of  lime  which  sometimes  have  a  small  grain  of  sand  as  a  nucleus. 
Limestones  are  also  at  times  composed  in  great  part  of  the 
shells  of  minute  animals  called  'foraminifera.'  These  organisms, 
whose  remains  constitute  the  very  earliest  record  of  life  of  which 
we  have  any  knowledge,  have  peopled  the  waters  of  various 
geological  epochs  with  their  descendants,  and  at  the  present 
day  the  foraminifera  have  numerous  living  representatives. 
The  animals  themselves  are  little  more  than  small  shapeless 
masses  of  animated  jelly,  but  they  have  the  power  of  sepa- 
rating  carbonate  of  lime  from   solution  in  water,   and  of 
building  up  the  material  into  shells  of  very  variable    and 
extremely  beautiful  forms.    Some  are  perforated  by  immense 
numbers  of  minute  holes  through  which  the  gelatinous  occu- 
pants can  protrude  their  filamentous  processes.     To  these 
holes,  or  foramina,  the  order  owes  its  name.     Corals  also 
have  the  power  of  secreting  large  quantities  of  carbonate  of 
lime,  and  some  limestone  rocks  are  in  great  part  due  to  the 
secretions  of  these  polyps.    The  shells  of  the  mollusca,  which 
have  originated  from  a  similar  secretive  faculty,  also  at  times 
contribute  largely  towards  the  formation  of  some  limestones. 
This  secretive  process  can  merely  be  regarded  as  a  chemical 
process  performed  through  the  intervention  of  the  animal; 
and  when  we   speak  of  such  rocks  as  having  an   organic 
origin,  we  must  be  careful  not  to  imply  that  the  animal  had 
actually  formed   the   calcareous  matter  instead   of  having 
merely  secreted  it.     Pure  limestones  consist  simply  of  car- 
bonate of  lime.     A  compound  of  the  carbonates  of  lime  and 
magnesia  constitutes    magnesian    limestone,   or  dolomite. 
Those  limestones  which  contain  a  certain  amount  of  clayey 


Stratified  Rocks.  19 

matter  are  termed  argillaceous  limestones,  and  those  con- 
taining sandy  impurities  are  styled  arenaceous  limestones. 

The  changes  which  sedimentary  rocks  undergo  may  be 
regarded  as  physical,  as  chemical,  or  as  the  result  of  phy- 
sical and  chemical  agencies  acting  either  simultaneously  or 
at  different  periods. 

CLASSIFICATION  OF  THE  SEDIMENTARY  ROCKS. 

Clay.  Composed   of  hydrous   silicate  of  alumina, 

usually  with  mechanical  admixture  of 
sand,  iron  oxides,  and  other  substances. 

Marl  Clay  containing  calcareous  matter. 

Shale.  Indurated  clay,  fissile  in  direction  of  bed- 

ding. 

Slate.  Indurated  clay,  fissile  in  parallel  planes  other 

than  those  of  bedding. 

f  Sand.  Chemical  composition  silica.     Mineral  com- 

ponents quartz  or  flint. 

Sandrock*      Coherent  sand. 

Sandstone.  A  more  or  less  strongly  coherent  and  often 
highly  indurated  sand. 

Grit.  A  coarse-grained  and  somewhat  coherent,  or 

at  times  a  fine-grained  and  very  hard  and 
compact  sandstone,  frequently  containing 
fragments  and  granules  of  other  minerals 
beside  quartz,  flint,  or  chert. 

Calcareoiis  Sandstone.  Sandstone  cemented  by  carbo- 
nate of  lime. 

Ferruginous  Sandstone.  Sandstone  cemented  by  an 
oxide  of  iron  or  by  carbonate  of  iron. 

Conglomerate.  Rounded  pebbles  of  flint,  chert,  jasper, 
quartz,  &c.,  cemented  either  by  siliceous, 
calcareous,  or  ferruginous  matter. 

Siliceous  Breccia.  A  rock  similar  to  the  above  in  com- 
position, but  differing  from  it  in  contain- 
ing angular  fragments  instead  of  rounded 
pebbles. 

Siliceous  Sinter.     Silica    deposited   in   a   more   or  less 
loose  or  spongy  form  from  waters  holding 
silica  in  solution, 
c  2 


2O  The  Rudiments  of  Petrology. 

Many  of  the  slates,  sandstones,  and  grits  afford  good 
building  stones. 

Limestone.  Chemical  composition,  carbonate  of  lime. 
Limestones  vary  greatly  in  their  physical 
characters ;  some  being  earthy,  soft,  and 
friable,  as  chalk ;  others  hard  and  crys- 
talline. 

The  principal  limestones  used  for  building 
purposes  are  the  Devonian,  the  carboni- 
ferous, and  the  magnesian  limestones  ; 
many  of  the  oolitic  limestones,  especially 
the  Bath  stone,  Portland  stone,  and  Pur- 
beck  limestone. 

Limestones  which  are  capable  of  receiving 
a  polish  are  called  marbles.  They  vary 
so  greatly  that  it  is  not  possible  to  describe 
even  the  leading  kinds  in  a  -small  space. 
Bands  of  chert  occur  in  the  carboniferous 
and  in  some  other  limestones,  as  the  Port- 
land, and  bands  and  nodules  of  flint  are 
•met  with  in  the  upper  chalk. 

Magnesian  Limestone.  Chemical  composition,  carbo- 
nates of  lime  and  magnesia.  This  rock  is 
also  called  Dolomite,  after  Dolomieu. 
The  proportions  of  carbonate  of  lime  to 
carbonate  of  magnesia  vary  greatly  in 
different  localities. 

Argillaceous  Limestone.  Limestone  containing  some 
•clayey  matter  or  hydrous  silicate  of  alu- 
mina. When  this  reaches  a  certain  pro- 
portion the  rock  is  termed  an  hydraulic 
limesitone  ;  such  limestones  are  used  for 
the  manufacture  of  cements  which  set 
under  water  (hydraulic  cements).  The 
lias  limestone  is  a  good  example  of  an 
argillaceous  limestone. 

Arenaceous  or  Siliceoiis  Limestones  represent  a  transi- 
tional condition  between  limestone  and 
chert.  Some  of  them,  such  as  the  Kentish 
rag,  afford  good  building  stones. 


Dip  and  Strike. 


21 


It  has  been  already  stated  that  the  sedimentary  rocks 
occur  in  beds  or  strata  (hence  they  are  also  called  stratified 
rocks).  This  arrangement  has,  in  the  first  instance,  been  an 
approximately  horizontal  one,  and,  in  most  cases,  where 
there  is  any  marked  deviation  from  horizontally,  the 
deposits  have  been  disturbed  by  the  action  of  subterranean 
forces.  When  any  such  disturbance  has  taken  place,  so  as 
to  communicate  an  inclination  to  the  beds,  this  inclina- 
tion is  termed  '  dip.'  If  we  assume  a  long  strip  of  paper  to 
represent  a  horizontally  deposited  bed  or  stratum,  and  then 
fold  it  lengthwise  as  in  FlG  2 

fig.  2,  we  have  two 
dips,  one  in  the  direc- 
tion of  a  and  the  other 
in  an  opposite  direction,  b.  The  direction  be  or  cb  is 
termed  the  '  strike '  of  the  beds.  The  strike  is  always  an 
assumed  horizontal  direction,  so  that  if  we  tilt  our  strip  of  paper 
on  one  end  the  strike  will 
still  be  a  horizontal  line 
as  de  (fig.  3).  The  dip  is 
always  reckoned  at  right 
angles  to  the  strike.  It 
is  somewhat  difficult  to 
render  this  apparent  in  a  diagram  ;  but  if  we  represent  one 
side,  of  our  strip  of  paper  to  be  dipping  vertically,  i.e.  at  90°, 
as  in  fig.  4,  it  will  render  the  FIG 

mutual  directions  of  dip  and 
strike    as    seen   in   plan :    a  b          ^^         J 
representing  the  strike,  and  cd  & 

the  dip.     In  nature  it  is  not 

usual  to  find  beds  bent  in  the  acute  manner  indicated  in 
the  preceding  figures.  They  generally  describe  curves 
which  represent  the  arcs  of  circles  sometimes  many  miles, 
sometimes  only  a  few  feet  in  extent.  In  the  latter  case 
this  small  crumpling  is  spoken  of  as  '  contortion.'  When 
flexure  of  strata  occurs  in  an  upward  direction  the  result 


FIG.  3. 


The  Rudiments  of  Petrology. 


FIG.  5. 


is  spoken  of  as  an  anticlinal  flexure,  curve,  or  ridge  ;  while, 
on  the  other  hand,  when  the  curve  is  directed  downwards  in 
a  basin -shaped  manner,  it  is  termed  '  synclinal.  \ 

The  dip  of  strata  exercises  a  marked  influence  on  the 
scenery  of  a  country.  If  no  disturbance  of  stratified  deposits 
had  ever  taken  place  in  a  district  no  knowledge  of  its 
geology  could  be  obtained,  except  along  valleys  which  had 
been  scooped  out  by  the  action  of  rain,  rivers,  and  general 
atmospheric  agency,  or  in  railway  cuttings,  quarries,  and 
other  excavations,  and  in  borings  such  as  wells  and  the 
shafts  of  mines. 

Let  AB  (fig.  5)  represent  the  surface  of  a  country  in  which 
the  strata  have  never  been 
disturbed*  and  therefore  lie 
horizontally  just  as  they  were 
deposited.  Let  us  also  sup- 
pose that  the  surface  has  not 

;  -  •;  /.'.•.•.  •:.•>:".:  •;          been    carved    out  into    hills 

and  valleys,  but  is  a  level,  un- 
broken surface. 

It  is  evident  that  an  observer  walking  across  such  a 
district  would  meet  with  no  diver- 
sity in  the  character  of  the  soil  or 
of  the  rocks  over  which  he  passed, 
unless  indeed  the  same  deposit  ex- 
hibited slight  lithological  change  in 
its  own  horizon — such  as  a  passage 
from  clay  into  sandy  clay,  and  the 
geological  map  of  the  district  repre- 
sented by  the  section  A  B  would,  if 
coloured,  be  merely  painted  over 
with  one  uniform  tint,  or  stippled 
thus  (fig.  6). 

If  such  a  country  were  scooped  out  by  the  action  of 
rain,  rivers,. &c.,  the  section  AB  (fig.  5)  would  undergo  con- 
siderable alteration,  as  shown  in  the  accompanying  fig.  7  ; 


FIG.  6. 


Geological  Maps  and  Sections. 


FIG.  8. 


while  its  geological  map  would  now  indicate  the  exposure 
not  merely  of  the   uppermost 
stratum,  but  also  the  outcrop  of 
several  underlying  beds  some- 
what in  the  manner  shown  in          _____ 

fig.  8,  with  probably  a  river  R  R     \".=' ?  ".'~Vr'""T""~?'T. "  vr^        --. 
running  along  the   bottom  of 
the  valley. 

Maps  of  the  oolitic  and  liassic  districts  of  England  will 
be  seen   to   resemble  this  in  their 
geological  boundary  lines,  and  these 
lines  usually  follow  the  general  con- 
tours of  the  country. 

Let  AB  (fig.  9)  represent  the 
surface  of  a  country  in  which  the 
stratified  rocks  have  been  disturbed 
and  tilted  upwards  ;  in  other  words, 
a  country  in  which  the  strata  have 
a  definite  dip. 

It  is  highly  improbable  that 
such  a  country  would  have  a  level 
surface,  as  the  unequal  hardness  of  the  different  rocks 
exposed  to  the  action  of 
the  atmosphere  would  tend 
to  beget  considerable  irre- 
gularity, the  harder  ones 
being  worn  away  less  easily 
than  the  soft  ones.  Still,  supposing  the  surface  of  the 
country  to  have  been  planed  off  to  a  perfect  level,  the 
geological  map  of  the  district  would  nevertheless  pre- 
sent great  diversity  in  its  colouring  or  shading.  A  man 
walking  across  the  country  from  east  to  west  would  pass 
over  several  different  lormations.  In  this  case  ss,  ss 
(fig.  10)  would  represent  the  strike  of  the  beds,  and  the 
arrows  would  show  the  direction  of  tlieir  dip,  at  right  angles 
to  the  strike. 


FIG.  9. 


i   B 


The  Rudiments  of  Petrology. 

If  the   beds  were   repeated,  dipping    in    an    opposite 
FlG.  I0.  direction,  i.e.  if  they  had  an  an- 

ticlinal arrangement,  some  esti- 
mate could  be  formed  of  the 
amount  of  rock  which  had  been 
denuded  by  restoring  the  curves 
as  indicated  by  the  dotted  lines 
in  fig.  n,  although  this  would 
probably  represent  only  a  portion 
of  the  total  amount  of  matter 
which  had  been  removed. 

The  thickness  of  beds  should 
in  all  cases  be  measured  at  right 
to  the  planes  of  bedding,  whether  they   be  undis- 

FlG.   II. 


turbed  and   horizontal  or  disturbed  and   inclined.     Thus, 

if  A  A  (fig.  12)  represent  the 
surface  of  the  ground,  and 
B  an  inclined  bed,  then  the 
thickness  of  B  should  be 
measured  along  the  dotted 
line  xx,  and  not  along  the 
surface  A  A. 

Enough  has  now  been  said  to  denote  the  way  in  which 
the  disturbance  of  sedimentary  rocks  influences  the  surface 
of  a  country,  and,  when  aided  by  denudation,  promotes  our 
knowledge  of  its  geology,  by  bringing  to  view  subjacent 
deposits  which  would  otherwise  have  been  accessible  only 
by  excavation  and  boring,  thus  affording  us  the  means 
of  selecting  from  various  sources  materials  of  industrial 


Denudation.  2  5 

importance —  such  as  building  stones  ;  bringing  within  work- 
able distance  various  mineral  deposits,  and  diversifying 
the  surface  of  the  land  in  a  manner  which  affects  agri- 
culture and  water-supply,  civil  engineering,  and  last,  but 
not  least,  the  sanitary  condition  of  its  inhabitants. 

The  manner  in  which  rocks  are  worn  away  is  spoken  of 
as  denudation.  Denudation  may  be  regarded  under  two 
heads  : 

(1)  Marine  denudation. 

(2)  Atmospheric  or  subaerial  denudation. 

The  tendency  of  all  denudation  is  to  wear  away  existing 
land  to  lower  levels  until  it  reaches  the  level  of  the  sea. 
The  wearing  process  then  stops,  because  the  sea  can  only 
act  destructively  in  planes  situated  between  high  and  low 
water-mark.  The  breakers  do  all  the  work  of  marine  denu- 
dation ;  and  when  they  can  no  longer  act  upon  rocks 
because  they  have  planed  them  down  so  far  as  they  can 
plane  them,  that  is,  to  their  own  level,  the  process  of  denu- 
dation of  course  ceases ;  and  when  such  a  stage  of  degrada- 
tion is  reached,  subaerial  denudation  also  becomes  inert. 
Marine  denudation  may  therefore  be  denned  as  the  de- 
grading influence  exercised  by  the  sea  on  a  level  with  the 
breakers.  This  degradation  is  effected  not  merely  by  the 
force  of  the  breakers  dashing  and  pounding  against  their 
barriers,  but  the  detached  fragments  are  hurled  again  and 
again  against  the  rOcks,  which  thus  continually  supply 
ammunition  for  their  own  destruction.  The  tendency  of  all 
this  is  to  cut  back  the  coast  into  cliffs ;  and  the  process,  so 
far  as  the  sea  is  then  concerned,  becomes  an  undermining  one. 
This  is  well  shown  in  Shakespeare's  Cliff  at  Dover,  where 
the  sea  has  undercut  the  chalk  at  the  base  of  the  cliff 
(fig.  13) ;  but  it  is  usual  to  see  few  signs  of  excavation  at  the 
foot  of  a  cliff,  because  subaerial  denudation  is  also  con- 
tinually going  on ;  and  when  its  effects  upon  certain  rocks 
are  more  marked  than  those  of  marine  denudation,  the  face 
of  the  cliff  will  be  seen  to  slope  backwards  more  or  less. 


26 


The  Rudiments  of  Petrology. 


If  marine  denudation  acted  more  powerfully  than  atmospheric 
denudation,  then  the  cliffs  would  become  undermined,  and 
the  overhanging  masses  would  eventually  give  way,  falling 
on  to  the  shore  and  forming  a  talus,  or  heap  of  broken  frag- 
ments banked  up  against  the  lower  portion  of  the  cliffs.  A 
talus  may  also  be  formed  by  the  fall  of  fragments  dislodged 
by  frost  and  other  atmospheric  denuding  agents,  so  that 
under  any  circumstances  it  is  common  to  find  the  foot  of  a 
cliff  so  protected,  except  where  the  shore  is  very  steep 
or  where  the  scour  of  the  tide  is  considerable. 


FIG. 


These  natural  barricades  prevent  the  sea  from  attacking  the 
base  of  the  cliff  for  a  time,  but  after  a  while  they  are  cleared 
away  and  the  undermining  process  recommences,  to  be 
again  temporarily  retarded  by  successive  falls  of  rock  from 
above.  When  the  stratification  of  the  rocks  which  form  a 
coast  dips  inland,  the  sea  acts  very  destructively,  large  frag- 
ments being  dislodged  with  comparative  ease ;  but  when  the 
dip  is  seawards,  the  breakers  run  up  inclined  planes  of 
bedding  instead  of  dashing  against  an  abruptly  raised  barrier, 
and  so  the  work  of  degradation  takes  place  much  more 


Cliffs  and  Escarpments. 


27 


slowly.  Sometimes  steep  slopes,  forming  features  which 
to  a  certain  extent  resemble  cliffs,  are  seen  far  inland.  These 
are  called  escarpments.  They  owe  their  origin  to  subaerial 


FIG.  14. 


/  >• 


and  not  to  marine  denudation,  and  they  differ  from  true 
sea-formed  cliffs  in  that  the  escarpment  will  be  found  to 
trend  in  a  direction  parallel  to  the  strike  of  the  beds  (fig.  14), 

Flc.  15, 


FIG.  1 6, 


'while  a  coast  line  backed  by  cliffs  runs  in  an  irregular 
manner,  which  bears  no  relation  whatever  to  the  strike 
(fig.  15).  It  is  therefore  evident 
that  the  configuration  of  a  coun- 
try, so  far  as  its  coast-line  is 
concerned,  does  not  depend 
upon  the  strike  of  the  rocks 
along  that  coast -line  except  so 
far  as  the  strike  determines  the 
position  along  that  coast-line,  of 
relatively  hard  and  soft  rocks, 
or  of  rocks  whose  chemical  or  physical  characters  render 


28  The  Rudiments  of  Petrology. 

them  relatively  difficult  or  easy  of  disintegration,  as  in 
fig.  1 6,  where  H  represents  rocks  comparatively  difficult,  and 
E  those  which  are  comparatively  easy,  to  wear  away.1 

The  difference  in  the  weathering  of  rocks,  dependent 
upon  their  relative  hardness,  does  not  merely  influence  the 
coast-line  of  a  country,  but  it  affects  its  inland  configuration 
to  a  greater  or  less  extent,  as  shown  on  the  east  and  west 
coasts  of  England.  lfr  for  example,  a  country  consist  partly 
of  granite  and  partly  of  slate,  it  will  usually  be  found  that 
the  granite  constitutes  the  high  ground,  while  the  slates 
occupy  the  lower  portions  of  the  district.  It  does  not,  how- 
ever, necessarily  follow  that  all  the  valleys  of  a  country 
should  be  scooped  out  in  the  softer  rocks,  while  the  harder 
ones  only  form  the  hills.  If  this  were  the  case,  the  drainage 
of  a  country  would  be  determined  by  the  general  strike  of 
the  rocks,  and  all  the  valleys  would  trend  in  the  direction  of 
the  strike.  This,  however,  is  not  always  the  case.  In  the 
Lake  District  of  England,  for  example,  most  of  the  valleys 
run  across  the  general  strike  of  the  Upper  Silurian  rocks, 
instead  of  coinciding  with  it.  It  seems,  indeed,  that  the 
directions  of  the  rivers  and  valleys  of  a  country  are  often 
determined  rather  by  its  initial  slope  than  by  the  relative 
resistances  offered  to  erosion  by  its  rocks.  As  a  rule,  how- 
ever, it  is  probable  that  neither  of  these  considerations  can 
be  utterly  ignored,  and  that  the  truth  involves  them  both  : 
first,  the  initial  slope  of  the  district ;  and  secondly,  the  relative 
resistance  which  the  rocks  offer  to  atmospheric  denudation. 
Such  resistance  does  not  merely  influence  the  large  features 
of  a  country  ;  it  renders  itself  evident  in  the  actual  shapes 
assumed  by  the  hills. 

1  The  Geological  Observer  and  How  to  Observe  Geology,  by  Sir  Henry 
De  la  Beche,  are  good  works  for  the  student  to  consult  upon  these 
points ;  also  a  Paper  on  '  Subaerial  Denudation  and  on  the  Cliffs  and 
Escarpments  of  the  Chalk  and  the  Lower  London  Tertiary  Beds,'  by 
W.  Whitaker,  Geol.  Mag.  vol.  iv.  pp.  447-483.  London.  1867.  The 
phenomena  of  denudation  are  also  well  described  in  Prof.  Ramsay's 
Physical  Geology  and  Geography  of  Great  Britain. 


Weathering  of  Rocks.  29 

The  forms  of  hills  and  mountains  are  mainly  due — 

A.  If  composed  of  sedimentary  rocks — 

(1)  To  the  lie  of  the  beds,  whether  horizontal  or  in- 

clined. 

(2)  To  the  presence  or  absence,  the  paucity  or  multitude, 

of   the    structural    planes   which    traverse    those 
beds. 

(3)  To  the  physical  characters  of  the  beds. 

(4)  To  their  chemical  composition. 

B.  If  composed  of  eruptive  rocks — 

(1)  To  the  presence  or  absence,  paucity  or  profusion,  of 

structural  planes, 

(2)  To  the  physical  character  of  the  rocks, 

(3)  To  their  mineral  constitution. 

(4)  To  their  chemical  composition. 

The  average  rainfall  of  a  district  of  course  has  more 
or  less  influence  on  the  weathering  and  disintegration  of 
rocks,  while  the  water  which  filters  into  them,  especially 
along  joint  planes  and  fissures,  expands,  on  its  conversion 
into  ice,  in  frosty  weather,  and  greatly  facilitates  their 
degradation,  by  forcing  apart  fragments  and  blocks  of  rock 
which  become  detached  as  soon  as  the  ice  thaws.  The 
process  of  disintegration  is  carried  on  to  a  great  extent  by 
the  rain,  which,  in  its  passage  through  the  atmosphere,  ab- 
sorbs a  considerable  quantity  of  carbonic  acid.  This  acts 
upon  compounds  of  lime,  potash,  soda,  &c.  In  limestone 
districts  a  great  amount  of  matter  is  annually  removed 
in  this  way  by  the  conversion  of  the  carbonate  into  the 
soluble  bicarbonate  of  lime.  The  water  of  rivers,  and  the 
water  from  swamps  and  peat-mosses  also,  contains  more 
or  less  carbonic  acid,  especially  in  the  latter  cases,  where 
vegetable  matter  by  its  decomposition  gives  off  considerable 
quantities  of  this  gas.  Limestones  are  by  no  means  the 
only  rocks  acted  upon  by  waters  containing  carbonic  acid. 


30  The  Rudiments  of  Petrology. 

The  felspars,  which  constitute  so  great  a  portion  of  many 
eruptive  rocks,  suffer  decomposition  through  this  cause.  If 
the  felspar  be  a  lime-felspar  the  lime  is  the  substance  first 
removed;  the  potash  and  seda,  which  most  felspars  contain, 
are  next  carried  off  as  carbonates,  although  they  are  some- 
times removed  in  the  form  of  soluble  silicates.  This  gradual 
removal  of  the  alkalies,  &c.,  ultimately  results  in  the  forma- 
tion of  kaolin  or  china-clay,  a  hydrous  silicate  of  alumina, 
2^iSi2+2H2-  Should  the  waters  contain  magnesian  salts  in 
solution,  the  lime  or  soda  of  the  felspars  may  be  replaced 
by  the  isomorphous  magnesia,  thus  giving  rise  to  steatitic 
matter  so  long  as  the  alumina  of  the  felspar  is  not  involved 
in  the  change.  Through  similar  causes  the  various  species  of 
mica  become  at  times  partially  decomposed  and  converted 
into  steatite,  serpentine,  compounds  allied  to  chlorite,  and 
possibly  some  other  hydrated  minerals.  The  alterations 
and  replacements  of  minerals,  by  the  infiltration  of  water 
charged  with  acids  and  soluble  salts  through  the  rocks  in 
which  they  occur,  are  so  numerous,  and  the  pseudomorphs  to 
which  they  give  rise  are  so  interesting,  that  special  mention  of 
them  will  be  made  in  those  parts  of  this  work  where  the  rocks 
in  which  they  are  found  are  described.  The  interchanges 
which  take  place  in  the  formation  of  these  pseudomorphous 
minerals  are  in  great  part  due  to  the  isomorphism  of  the 
replaced  and  the  replacing  substances,  although  at  times  the 
original  mineral  may  be  entirely  removed  and  its  place 
subsequently  occupied  by  matter  bearing  no  such  relation 
to  the  components  of  the  mineral  replaced. 

It  has  already  been  stated  that  the  sedimentary  rocks  occur 
in  layers,  technically  termed  strata  or  beds.  A  number  of 
such  beds,  deposited  at  the  same  time,  under  approximately 
the  same  conditions,  in  one  or  in  many  distinct  areas  (areas 
which  during  deposition  may  once  have  been  united,  or  in 
which  deposition  may  have  gone  on  independently),  are 
termed  '  formations.'  The  formations  are  usually  more  or 
less  fossiliferous,  and  separate  formations  are  distinguished 


Formations.  *j*  3* 

'//.     v  , 

by  characteristic  fossils,  which  bear  testimony  to  A<  certain'/  . 
sameness  in  the  animal  and  vegetable  lif^/vhich 'exjsted 
from  the  deposition  of  the  lowest  to  that  of  the  highest  beds 
in  the  formation.  The  geological  age  of  any  bflfc^may'  ^ 
therefore  be  determined  either  by  its  stratigraphical  fi6ri^6ji 
or  by  the  fossils  which  it  contains.  The  lithological  cha-y 
racter  of  a  bed  sometimes  varies,  and,  therefore,  less  depen- 
dence is  to  be  placed  upon  it  than  upon  palaeontological 
evidence.  Furthermore,  the  slight  variation  in  the  litho- 
logical characters  of  sedimentary  rocks  often  renders  it  very 
difficult  to  assign  them  to  any  particular  horizon  in  the 
absence  of  fossils ;  sandstones,  slates,  shales,  and  limestones 
of  different  geological  ages  often  bearing  a  close  resemblance 
to  one  another.  Again,  rocks  differing  widely  in  lithological 
character  may  have  been  deposited  at  the  same  time,  as  in 
the  case  of  the  Devonian  and  old  red  sandstone  rocks,  the 
former  having  been  thrown  down  in  the  sea,  and  the  latter 
in  lakes,  as  proved  by  the  fossils  which  they  respectively 
contain.  Yet  both  formations  occupy  a  position  intermediate 
between  the  Upper  Silurian  rocks  and  the  lowest  members 
of  the  carboniferous  series.  The  grouping  of  sedimentary 
rocks  into  formations  is,  of  course,  more  or  less  arbitrary. 
Some  genera,  and  frequently  species,  which  occur  in  a  lower 
formation  are  often  represented  in  the  succeeding  deposits 
of  a  newer  formation,  and  probably,  if  the  truth  were  known, 
it  would  be  found  that  all  the  formations,  which  we  now 
recognise,  pass  from  one  into  another.  Because  an  uncon- 
formity, i.e.  a  break  both  stratigraphical  and  organic,  occurs 
in  one  limited  district,  it  does  not  necessarily  follow  that  this 
break  extended  over  the  entire  globe.  Allowances  must  be 
made  for  relative  distributions  of  land  and  water,  which  we 
have  often  no  means  of  realising,  and  no  doubt  the  universal 
application  of  limited  knowledge  often  does  more  harm 
than  good  in  this  branch  of  geological  inquiry. 


32  The  Rudiments  of  Petrology. 

CHAPTER  IV. 

ERUPTIVE   AND    METAMORPHIC    ROCKS. 

THE  eruptive  or  igneous  rocks  differ  entirely  from  those  of 
sedimentary  origin  in  their  mode  of  occurrence  (except  in 
the  case  of  volcanic  ejectamenta,  presently  to  be  explained, 
in  the  interbedding  of  lava-flows,  and  in  the  intrusion  of 
sheets  of  eruptive  rock  between  planes  of  bedding,  as  in  the 
case  of  the  Whin  Sill  of  Northumberland * ).  They  bear,  except 
in  such  instances  as  those  just  cited,  no  definite  relation 
to  the  sedimentary  rocks,  but  form  irregular  masses,  often  of 
very  great  extent,  from  which  vein-like  prolongations  or 
tabular  and  wall-like  masses  (dykes)  are  often  sent  off  into 
the  surrounding  rocks.  They  also  emanate  from  volcanic 
vents  in  the  form  of  molten  viscous  lava,  forming  flows  or 
coulees,  which  are  forced  over  the  edge  of  the  crater, 
frequently  breaking  it  down  on  one  side,  creeping  down  the 
sides  of  the  cone,  and  often  spreading  for  many  miles  over 
the  surrounding  country.2 

Sometimes  they  are  erupted  in  the  form  of  large  and 
small  fragments  of  rock  (lapilli),  of  peculiar  spheroidal 
molten  masses  (volcanic  bombs),  and  of  finely  comminuted 
and  dusty  mineral  matter  (ashes).  These  fragmentary  ejecta- 
menta are  often  thrown  high  into  the  air.  Part  of  them  fall 
back  again  into  the  crater  to  be  again  and  again  thrown  up, 
so  that  by  constant  attrition  they  become  more  or  less 
rounded.  Part  fall  on  and  outside  the  rim  of  the  crater, 
thus  helping  to  build  it  up  higher.  Part  may  be  carried  by 

1  Vide  paper  'On  the  Intrusive  Character  of  the  Whin  Sillof 
Northumberland,'  by  W.  Topley  and  G.   A.  Lebour,  Q.  J.  G.  S.,  vol. 
xxxiii.  p.  406. 

2  Intrusive  sheets  may  be  distinguished  from  true  lava- flows,  which 
have  been  subsequently  overlaid  conformably  by  sedimentary  strata,  by 
the  fact  that  the  rocks  both  above  and  below  the  intrusive  sheets  are 
altered  at  the  contacts,  while  in  the  case  of  lava-flows  the  rocks  over 
which  they  ran  have  been  altered,  but  the  deposits  above  them  show 
no  trace  of  metamorphism. 


Plutonic  and  Volcanic  Rocks.  33 

the  wind  and  showered  down  over  the  adjacent  country,  and 
if  in  the  state  of  very  fine  dust  may  be  transported  immense 
distances  by  the  wind,  a  passage  of  between  700  and  800 
miles  having  been  recorded.  There  are  also  some  craters, 
usually  of  comparatively  small  dimensions,  which  pour  out 
liquid  mud,  frequently  accompanied  by  an  outpouring  of 
water  also.  The  water  is  in  some  cases  boiling,  in  others 
cold,  and  bitumen  has  also  been  seen  to  exude  from  some 
of  them.  The  hot  water  ejected  by  the  Geysers  of  Iceland, 
and  that  of  the  thermal  springs  of  Roto-Mahana,  near  Lake 
Taupo,  in  New  Zealand,  carry  a  large  amount  of  silica  in 
solution,  which  on  the  evaporation  of  the  water  leaves  a 
deposit  or  incrustation  of  white  siliceous  sinter.  Besides  the 
lavas,  ashes,  &c.,  which  emanate  from  volcanoes,  steam  and 
gases,  such  as  carbonic  anhydride,  sulphurous  acid,  hydro- 
chloric acid  gas,  sulphur  vapour,  &c.,  are  also  emitted. 

It  is  still  customary  to  divide  the  eruptive  rocks  into  two 
classes — the  plutonic  and  the  volcanic,  the  former  class 
including  those  rocks  which  have  solidified  at  considerable 
depths  beneath  the  earth's  surface,  and  which  are  now  only 
exposed  because  the  rocks  which  once  overlaid  them  have 
been  removed  by  denudation.  The  volcanic  rocks,  on  the 
other  hand,  although  likewise  originating  at  considerable 
depths,  have  been  forced  up  until  they  not  merely  reached 
but  in  many  cases  have  overrun  the  surface.  From  the 
mode  of  occurrence  of  the  rocks  belonging  to  these  two 
classes  it  is  by  no  means  easy  to  affirm  with  positive 
certainty  that  they  are  merely  different  phases  of  the  same 
eruptive  rock-forming  matter,  emanating  from  the  same 
source.  Still,  on  comparing  the  rocks  of  the  one  class  with 
those  of  the  other,  a  tolerably  continuous  chain  of  evidence 
can  be  adduced  to  show  that  they  graduate  into  one 
another,  and  that  this,  like  all  other  classifications,  which 
are  necessarily  arbitrary,  is  more  hypothetical  than  real. 
The  quartz-porphyries  or  elvans  resemble  the  granites  more 
or  less  in  mineral  composition,  and  are  known  to  emanate 

D 


34  The  Rudiments  of  Petrology. 

from  granitic  masses ;  but  the  mica,  which  is  plentiful  in 
granites,  is  only  poorly  represented  or  is  totally  absent  in  the 
quartz-porphyries.  The  porphyritic  felstones  resemble  the 
quartz-porphyries,  except  that  they  contain  no  definite  and 
well-marked  crystals  and  blebs  of  quartz.  The  felstones  which 
are  not  porphyritic  are  often  identical  with  the  magma  of  quartz- 
porphyry.  The  trachytes  vary  in  their  affinities,  some  (such 
as  the  quartz-trachytes)  inclining  more  towards  the  granites 
and  felstones  in  mineral  composition,  while  others  (as  sani- 
dine-oligoclase-trachytes)  occupy  an  intermediate  position, 
passing  into  or  becoming  allied  to  the  basalts,  dolerites, 
&c.,  in  the  oligoclase  trachytes,  the  andesites,  and  the 
trachy-dolerites.1  If,  then,  such  close  resemblances  in 
mineral  constitution  can  be  discerned  in  the  rocks  which 
come  near  the  boundary  line  drawn  between  the  two  classes, 
and  since  rocks  containing  under  60  per  cent  of  silica 
(basic,  of  Bunsen),  and  over  60  per  cent,  of  silica  (acidic,  of 
Bunsen)  are  found  in  both  classes,  it  seems  reasonable  to 
suppose  that  close  resemblances  in  mineral  constitution  are 
almost  equivalent  to  observed  passages.  Again,  both  acidic 
and  basic  rocks  are  known  in  some  instances  to  have 
emanated  at  different  periods  from  the  same  volcanic  vent, 
Durocher,  in  the  '  Annales  des  Mines,'  vol.  xi.,  1857,  enun- 
ciated the  theory  that  all  eruptive  rocks  have  been  derived 
from  one  or  other  of  two  magmas  which  occur  in  distinct 
zones  beneath  the  solid  crust  of  the  earth,  the  one  poor  in 
basic  materials,  but  containing  over  60  per  cent,  of  silica,  the 
other  rich  in  basic  matter,  but  holding  less  than  60  per  cent, 
of  silica.  The  former  magma  having  a  less  specific  gravity 
than  the  latter,  is  assumed  to  float,  as  it  were,  upon  it,  the 

1  Mr.  J.  Clifton  Ward  states  that  some  of  the  rocks  occurring  in 
the  English  Lake  District  are  intermediate  in  mineral  composition 
between  felstones  and  dolerites,  and  in  describing  them  he  designates 
them  'felsi-dolerites.' — Q.  y.  G.  S.t  vol.  xxxi.  p.  417.  In  examining 
some  of  the  eruptive  rocks  from  the  Silurian  districts  of  North  Wales 
the  author  has  met  with  similar  examples,  and  can  fully  endorse  Mr. 
Ward's  conclusions. 


Cleavage.  3  5 

difference  in  specific  gravity  of  the  rocks  derived  from  these 
magmas  being  from  one  and  a-half  to  twice  as  great  as  between 
oil  and  water.  Taking  all  these  things  into  consideration, 
we  may  be  justified  in  assuming  that  the  difference  between 
the  plutonic  and  volcanic  rocks  of  Bunsen's  acidic  class  lies 
wholly  in  the  fact  that  they  have  solidified  under  different 
conditions,  but  that  their  differences  do  not  sufficiently 
warrant  their  separation  by  a  hard  and  conventional  boun- 
dary-line, nor  debar  us  from  the  inference  that  they  may 
often  have  arisen  from  the  same  deep-seated  sources.  The 
same  may  be  said  of  Bunsen's  basic  class.  If  it  could  be 
demonstrated  that  they  have  done  so,  nothing  would  remain 
but  to  admit  that  the  plutonic  rocks  are  the  roots  and  stem, 
the  volcanic  rocks  the  branches  and  twigs,  of  a  great  petro- 
logical  system. 

The  sedimentary  rocks,  as  already  mentioned,  occur  in 
beds,  or  strata  which  were  originally  deposited  in  an  ap- 
proximately horizontal  manner.  Furthermore,  the  beds 
generally  exhibit  lamination  (or  still  finer  bedding).  The 
mineral  particles  of  which 

FIG.  17. 

these  rocks  are  composed 
are,  when  inequiaxial,  ar- 
ranged with  their  longer 
axes  parallel  with  the  la- 
mination or  bedding  (fig.  17),  in  which  BB  represents  the 
direction  of  the  planes  of  bedding.  Fig.  18  indicates  the 
change  of  direction  which 

r  IG.  To. 

these   minute  and  usually  £ 

microscopic  particles  as- 
sume when  the  rock  has 
undergone  great  lateral 
pressure.  A  fissile  struc- 
ture, called  cleavage,  is 
then  set  up  in  some  direc- 
tion other  than  that  of  the  bedding  planes  B  B  ;  as,  for  example, 
in  the  direction  c  c,  so  that  the  rock  splits  more  or  less 

D  2 


36  The  Rudiments  of  Petrology. 

readily  in  that  direction.  This  change  in  the  minute  struc- 
ture of  rocks  which  have  been  subjected  to  strong  pressure 
was  first  demonstrated  by  Sorby,  and  fully  explained  by 
D.  Sharp  in  the  '  Quarterly  Journal  of  the  Geological  So- 
ciety,' and  by  Professor  John  Phillips  in  'Report  of  the 
British  Association,  1856.'  It  also  affects  any  fossils  which 
the  rock  may  contain,  squeezing  and  distorting  them  to  a 
considerable  extent.  True  schistose  fission  and  slaty  cleavage 
are  seldom  or  never  met  with  in  rocks  of  eruptive  origin, 
except  sometimes  in  beds  of  volcanic  ash,  and  occasionally 
in  some  of  the  older  lavas,  as  shown  by  J.  Arthur  Phillips, 
neither  does  lamination  occur  in  eruptive  rocks,  but  a  structure 
slightly  resembling  it  is  often  to  be  noticed  in  sedimentary 
rocks  which  have  undergone  such  great  change  (meta- 
morphism)  that  they  approach  to,  or  are  identical  with,  true 
eruptive  rocks  in  their  mineral  constitution.  This  foliation 
consists  in  the  segregation  of  any  one  mineral  component  of 
the  rock  along  a  more  or  less  regular  plane,  and  the  result 
is  a  differentiation  of  the  rock  into  a  series  of  alternating 
layers  of  different  mineral  composition.  These  layers  are 
often  very  thin,  and  at  times  scarcely  to  be  discerned  with 
the  naked  eye.  Hornblende  schist,  for  example,  consists  of 
alternating  layers  of  hornblende  and  quartz ;  gneiss  of  layers 
of  quartz,  felspar,  and  mica.  Gneiss  may  therefore  be 
regarded  lithologically  as  a  foliated  granite.  Foliation  has 
often  been  found  to  coincide  with  the  original  planes  of  bed- 
ding, as  noticed  by  Ramsay,  Darwin,  Sterry-Hunt,  and  other 
observers,  but  this  is  not  invariably  the  case.  Metamorphic 
rocks  form,  as  it  were,  a  connecting  link  between  the  sedi- 
mentary and  the  eruptive  classes,  their  pseudo-eruptive 
characters  having  been  superinduced  by  the  contact  or 
proximity  of  highly  heated  eruptive  matter.  Thus,  where 
basalts  come  in  contact  with  limestones,  the  latter  frequently 
become  crystalline  for  some  distance  from  the  contact. 
Such  metamorphism  affects  rocks  sometimes  on  a  small, 
sometimes  on  a  large  scale,  occasionally  influencing  only  a 


Volcanoes.  37 

few  inches,  at  other  times  extending  for  miles.  It  consists 
sometimes  merely  in  physical,  at  others  in  chemical  and 
physical  changes,  which  frequently  involve  complicated 
atomic  interchanges  (chemical  reactions),  and  symmetrical 
molecular  rearrangements  (crystallogenesis) .  By  these 
means  minerals  are  developed  in  a  rock  which  it  did  not 
previously  contain ;  and  this  process  may  take  place  without 
any  accession  of  fresh  elementary  substances,  analyses  of 
the  unaltered  and  the  metamorphosed  rock  being  sometimes 
nearly  identical.  The  presence  of  hygrometric  water,  or 
quarry  water,  greatly  facilitates  such  changes  in  the  mineral 
constitution  of  rocks.  This  fact  is  very  ably  dwelt  upon  by 
Mr.  John  Arthur  Phillips  in  several  papers  on  the  petrology 
of  Cornwall  published  during  the  last  few  years  in  the  '  Quar- 
terly Journal  of  the  Geological  Society.'  These  matters  will, 
however,  be  more  fully  discussed  in  the  sections  specially 
devoted  to  the  changes  which  rocks  undergo. 

Although  many  admirable  descriptions  of  volcanoes  are 
to  be  found  in  most  manuals  of  geology  and  in  works 
specially  devoted  to  the  geology  of  volcanic  districts,1  yet  it 
may  be  well  to  give  here  a  brief  description  of  the  general 
structure  of  volcanic  vents. 

An  active  volcano  may  be  denned  as  a  passage  or  pipe 
which  affords  to  deep-seated  mineral  matter,  in  a  state  of 
fusion,  the  means  of  transmission  through  the  earth's  crust, 
and  of  egress  at  its  surface.  A  passive  or  extinct  volcano 
is  one  in  which  this  communication  is  obstructed,  either  by 
a  plug  of  solidified  lava,  or  by  accumulations  of  fragmentary 
matter,  a  dissipation,  temporary  or  permanent,  of  the  eruptive 
energy,  permitting  the  solidification  of  the  molten  matter. 
Should  an  augmentation  of  the  eruptive  force  occur,  the  plug 
will  either  be  shattered,  and  ejected  in  the  form  of  lapilli 

1  The  student  may  consult  the  works  of  Lyell,  Scrope,  Darwin, 
Daubeny,  De  la  Beche,  &c.  with  advantage ;  also  some  very  interest- 
ing papers  by  Prof.  J.  W.  Jucld,  entitled  '  Contributions  to  the  Siudy 
of  Volcanoes,'  published  in  the  Geological  Magazine,  and  the  chapters 
devoted  to  this  subject  in  Geology  for  Students'  by  Prof.  A.  H.  Green. 


38  The  Rudiments  of  Petrology. 

and  ashes,  or  re-melted  and  poured  out  as  lava,  but,  if  it  be 
unable  to  re-open  the  old  passage,  new  vents  may  be  pro- 
duced, either  within  or  without  the  lip  of  the  crater. 

Lava  transmitted  through  a  fissure  or  pipe  and  extruded 
at  the  surface  may  give  rise  to  hills  of  a  dome-shaped  cha- 
racter. Ashes  and  lapilli  ejected  from  a  vent  become  piled 
up  around  it,  and  in  time  form  a  conical  hill  on  what  may 
once  have  been  a  level  surface.  As,  however,  they  are  loose 
incoherent  deposits,  the  hill  will  gradually  acquire  a  slope 
at  which  they  are  no  longer  stable.  On  measuring  the 
superficial  inclination  of  hills  composed  of  such  ejectamenta, 
it  has  been  found  that  the  slope  is  usually  about  30°.  *  If  we 
pour  sand  upon  a  level  surface,  it  forms  a  complete  cone, 
but  the  loose  volcanic  materials  come  from  below  in  the 
first  instance,  and  since  the  pipe  of  the  volcano  is  open,  and 
ashes  and  lapilli  are  ejected  from  it  and  fall  around  it,  the 
cone  can  have  no  apex.  Some  of  the  ejected  matter,  which 
is  not  carried  away  by  the  wind,  showers  down  again  upon 
the  hill  and  around  the  orifice  of  the  vent,  but  the  law  which 
governs  the  stability  of  these  loose  accumulations  again  pre- 
vents them  from  resting  upon  a  very  steep  slope,  and  they 
are  found  to  dip  inwards  towards  the  orifice  as  well  as  out- 
wards down  the  slopes  of  the  hill.  The  boundary  line 
between  these  two  slopes,  which  of  course  represents  their 
greatest  altitude,  assumes  a  more  or  less  annular  form,  and  the 
inner  slopes  which  dip  towards  the  vent  constitute  a  cup- 
like  hollow,  termed  a  '  crater.'  Volcanoes,  however,  pour 
out  lava  as  well  as  eject  ashes,  and  these  phenomena  usually 
alternate.  Lava  in  a  viscous,  pasty  condition,  rises  through 
the  pipe  into  the  crater,  where,  after  perhaps  surging  up  and 
down  for  a  time  in  a  state  of  ebullition,  it  rises  to  the  lip  of 
the  crater  and  runs  over  it  down  the  sides  of  the  hill  and 
for  some  distance  over  the  adjacent  country.  Sometimes 
the  mass  of  molten  lava  carries  away  one  side  of  the 

1  Further  observations  upon  this  subject  have  been  made  by  Prof. 
J.  Milne  in  the  Geological  Magazine.     Decade  II.  vol.  v.  p.  337. 


Hints  on  Collecting.  39 

crater,  forming  a  great  breach  through  which  successive 
streams  of  lava  are  poured.  The  eruptions  of  lava  may 
be  succeeded  by  fresh  ejections  of  lapilli  and  ashes,  and 
these  again  may  be  followed  by  more  lava  streams,  the  hill 
eventually  consisting  of  stratified  fragmentary  accumulations 
with  interbedded  flows  of  lava.  Occasionally  the  lava  is  also 
forced  through  fissures  in  these  deposits,  forming  dykes  or 
wall-like  masses  which  intersect  them  in  various  directions, 
usually,  however,  assuming  a  somewhat  radiate  disposition 
around  the  cone.  When  the  volcano  has  done  its  work  as 
a  safety-valve  the  eruptions  may  cease  for  a  time,  and  the 
vent  may  become  plugged  in  the  manner  already  described. 
Should  a  fresh  eruption  occur,  it  may  force  a  new  vent. 
Ashes  are  again  showered  out,  lava  is  again  poured  forth, 
and  a  new  cone  is  erected  within  the  old  one,  or  little  cones 
and  fumaroles  are  formed  on  the  sides  of  the  hill  and  dotted 
over  the  surrounding  country. 


CHAPTER    V. 

THE   COLLECTING   AND    ARRANGEMENT    OF   ROCK    SPECIMENS. 

IN  collecting  specimens  of  rocks,  it  should  be  borne  in  mind 
that  small  pieces  of  compact  and  fine  grained  rocks  answer 
the  collector's  purpose  just  as  well  as  large  ones,  and  often 
belter,  should  he  have  but  a  limited  space  in  which  to  store 
them.  Small  specimens  are  easier  to  get  than  larger  ones, 
and  in  the  course  of  a  day's  work,  a  much  greater  number 
of  small  specimens  can  be  carried.  In  collecting  for  a  mu- 
seum, where  there  is  plenty  of  available  space,  of  course 
large  specimens  are  best.  About  5  inches  by  4  inches  square 
will  be  found  a  convenient  size  when  they  are  properly  dressed, 
but  if  it  should  not  be  advisable  to  dress  them  in  the  field, 
larger  pieces  should  be  collected,  as  it  frequently  happens 
that  a  roughly  broken  block  is  reduced  to  half  its  original 


40  The  Rudiments  of  Petrology. 

size  before  it  is  properly  dressed.  When  a  rock  presents  any 
large  structural  peculiarities,  it  will  of  course  be  necessary 
to  collect  proportionally  large  specimens  in  order  to  show 
the  structure  clearly.  For  ordinary  private  collections, 
specimens  about  4  in.  by  3  in.  or  3  in.  by  z\  in.  square  are 
convenient  sizes,  or,  if  space  be  very  limited,  pieces  about 
two  inches  square  will  suffice. 

A  hammer  with  a  tolerably  heavy  head  made  of  Swede-iron, 
with  steel  ends  welded  on  and  well  tempered,  but  not  so  highly 
as  to  be  brittle  even  when  used  on  the  hardest  rocks,  will  be  found 
to  be  best  suited  for  collecting.  The  shaft  should  not  be  less  than 
13  or  14  inches  in  length  ;  a  tough  wood  such  as  ash  answers  very 
well  for  this  purpose,  and  care  should  betaken  in  the  selection  of 
the  wood.  The  eye  into  which  the  shaft  is  fitted  ought  not  to 
be  less  than  i£  inch  in  length  by  at  least  \  inch  in  breadth,  and 
the  head,  which  should  have  one  end  wedge-shaped,  ought  to 
be  filed  away  slightly  around  the  under  opening  of  the  eye  to 
reduce  the  chance  of  breaking  the  shaft,  as  the  fracture  almost 
always  takes  place  just  under  the  head  of  the  hammer.  The 
author  has  been  in  the  habit  of  using  hammers  with  very  heavy 
heads  and  with  shafts  long  enough  to  serve  as  walking-sticks. 
Much  heavier  blows  can  be  struck  with  a  hammer  of  this  kind 
than  with  a  short-handled  one,  and  their  use  does  not  necessi- 
tate such  continual  stooping.  In  some  cases,  however,  a  short- 
shafted  hammer  has  its  advantages,  while  for  the  purpose  of 
dressing  specimens  one  with  a  very  long  shaft  is  perfectly 
useless.  When,  therefore,  a  long- shafted  hammer  is  taken  into 
the  field  it  is  well  also  to  carry  a  light  dressing  hammer.  Sh  rt- 
shafted  hammers  are  most  easily  carried  in  a  small  leathern 
frog  with  a  flap,  on  the  back  of  which  are  fixed  one  or  two  little 
vertical  straps  through  which  a  waist  belt  is  run  ;  and  this  belt 
can  also  carry  pouches  for  a  compass  and  a  clinometer.  A 
strong  canvas  bag  of  tolerable  capacity  is  necessary  for  carrying 
the  specimens  in,  and  it  should  have  a  little  tab  by  which  it  can 
be  loosely  attached  to  a  button  on  the  back  of  the  coat  to  pre- 
vent it  from  slinging  forward  when  the  wearer  stoops.  A  good 
supply  of  paper  should  also  be  carried  in  which  to  wrap  the 
specimens,  and  on  the  inside  of  each  wrapper  the  precise 
locality  from  which  the  specimen  is  derived  should  be  recorded. 


Hints  on  Collecting.  41 

These  may  seem  trivial  details,  but  neglect  of  them  often  causes 
disappointment  and  inconvenience.  With  regard  to  the  best 
shape  for  the  crushing  end  of  the  hammer-head,  some  prefer  it 
flat  and  square,  and  others  rounded.  When  it  is  slightly 
rounded  the  hammerer  is  less  liable  to  be  struck  by  splinters  of 
stone,  but  for  chipping  purposes  a  flat  square  face  is  best,  and 
the  dressing  hammer  should  always  have  one  such  termination. 

In  collecting,  one  of  the  first  and  most  important  things 
is  to  procure  specimens  which  are  unweathered  or  which 
have  suffered  as  little  as  possible  from  atmospheric  agency. 
Sometimes  it  so  happens  that  a  weathered  surface  of  rock 
shows  structural  peculiarities  which  are  especially  worthy  of 
note,  owing  to  the  different  power  which  its  component 
minerals  possess  of  resisting  disintegration  and  decomposi- 
tion. Interesting  specimens  of  this  kind  should  always  be 
collected  so  long  as  their  transport  will  not  lessen  the 
number  of  more  interesting  unweathered  specimens.  It  is 
only  from  the  latter  that  a  true  knowledge  of  the  normal 
mineral  and  chemical  composition  of  rocks  can  be  derived. 
The  writer  lays  especial  stress  upon  this,  as  he  has  at  times 
been  greatly  troubled  by  being  requested  to  determine  rocks 
from  badly  selected  specimens  in  an  advanced  stage  of  de- 
composition. Where  quarries  occur  there  is  no  excuse  for 
collecting  such  rubbish.  In  other  cases  it  is  often  a  matter 
of  difficulty  to  get  unweathered  samples,  and  sometimes  any 
specimen  is  better  than  none  ;  still,  as  a  rule,  the  collection 
of  weathered  chips  is  time  wasted,  and  it  is  far  better  to  take 
a  little  extra  trouble  in  order  to  get  good  and  typical  pieces. 

The  specimens  should  ultimately  be  dressed  with  a  small 
hammer,  the  piece  of  stone  being  held  in  the  palm  of  the 
left  hand,  while  with  the  right  successive  flakes  and  chips 
are  struck  off  by  sharp  blows  with  the  hammer.  When  very 
tough  rocks  are  operated  upon  in  this  way  it  is  by  no  means 
uncommon  for  the  novice  to  end  by  getting  a  more  or  less 
rounded  mass,  covered  all  over  with  powdery,  crushed  sur- 
faces, resulting  from  the  bruises  made  by  the  hammer,  and 
which  do  not  show  the  character  of  the  rock.  With  practice 


42  The  Rudiments  of  Petrology. 

he  will,  however,  soon  ascertain  the  directions  in  which  his 
blows  will  prove  effective,  and  those  where  no  amount  of  ham- 
mering will  avail.  When  the  specimen  is  properly  dressed, 
one,  or,  still  better,  two  little  labels  carrying  numbers  should 
be  affixed  to  it.  The  use  of  two  labels  is  desirable,  since,  if 
one  becomes  detached,  the  specimen  can  still  be  identified, 
so  long  as  the  other  adheres.  Ordinary  strong  gum  answers 
fairly  well,  but  in  some  cases,  especially  when  the  rock  is 
soft  and  earthy,  glue  will  be  found  preferable.  Specimens 
are  sometimes  numbered  with  red  sealing-wax  varnish,  but 
the  figures  are  often  difficult  to  find,  and,  when  painted  on 
rough  surfaces,  are  not  very  legible.  Of  course,  where  num- 
bers are  used,  the  specimens  must  be  carefully  catalogued. 
In  other  cases  labels  an  inch  or  more  in  length  may  be 
affixed,  with  the  name  and  locality  of  the  specimens  written 
on  them,  but  they  have  the  disadvantage  of  covering  a  larger 
space  than  the  little  numbered  tickets.  Should  labels  be 
used,  care  should  be  taken  to  stick  them  on  the  worst  dressed 
and  least  interesting  parts  of  the  specimens.  When  rocks 
are  very  soft  and  earthy,  numerals  may  be  scraped  on  them 
and  form  more  permanent  records  than  labels,  which  often 
become  detached,  even  without  handling.  In  arranging 
rock  specimens,  various  systems  of  classification  may  be 
adopted.  When  they  are  intended  to  illustrate  the  petrology 
of  any  particular  district  or  country  they  have  merely  a 
topographical  arrangement  which  seldom  admits  of  any 
really  scientific  classification,  since  eruptive  and  metamor- 
phosed rocks  have  to  be  placed  beside  the  sedimentary  ones 
with  which  they  are  associated,  and,  so  far  as  eruptive  rocks 
are  concerned,  this  does  not  always  mean  a  chronological 
arrangement.  The  latter  is  certainly  the  right  system  to 
follow  in  dealing  with  sedimentary  rocks,  but  with  those  of 
eruptive  origin  it  is  often  very  uncertain,  and  is  of  compara- 
tively little  value,  at  all  events,  in  the  present  state  of  our 
knowledge ;  since  eruptive  rocks,  almost  identical  in  mineral 
composition,  range  from  very  early  geological  periods  up  to 


Arrangement  of  Petrological  Collections.         43 

the  most  recent  times.  For  the  purpose  of  teaching 
petrology  a  classification  based  upon  the  mineral  constitu- 
tion of  the  specimens  is  doubtless  the  best,  although,  for 
general  geology,  the  topographical  arrangement  is  a  useful 
one.  When  this  is  adopted  in  museums  there  should  also 
be  another  small  collection  of  rocks  classified  according  to 
their  mineral  composition.  A  classification  based  upon 
structure  is  also  to  some  extent  to  be  commended,  since  it 
serves  more  or  less  as  a  grouping  according  to  the  condi- 
tions under  which  the  rocks  have  been  formed.  Collections 
so  arranged  as  to  be  illustrative  of  the  stones  used  for  build- 
ing and  ornamental  purposes  also  have  their  advantages, 
but  the  former  should  always  include  weathered  examples 
of  the  rocks  to  illustrate  iheir  powers  of  resisting  disintegrat- 
ing agencies,  and,  with  this  end  in  view,  it  would  be  most 
desirable  to  have  an  accompanying  suite  of  weathered  and 
partially  decomposed  stones  taken  from  buildings,  with  the 
date  when  the  building  was  erected  recorded  on  them,  or, 
if  they  have  merely  been  used  in  the  restoration  of  those 
edifices,  the  date  of  those  repairs  should  be  affixed  to  them, 
to  show  how  much  disintegration  the  stone  has  undergone 
since  its  surface  was  dressed  and  exposed  in  the  building. 
In  private  collections,  where  rock  specimens  are  usually 
arranged  in  cabinets,  the  drawers  should  not  be  less  than 
two  and  a  quarter  inches  in  depth  (inside  measure),  and 
deeper  drawers  are  often  very  convenient.  In  museums, 
where  space  is  ample,  table-cases  are  best  suited  for  the  dis- 
play of  the  specimens.  Wall-cases  are  objectionable,  because 
the  specimens  on  the  higher  and  lower  shelves  cannot  be 
seen  with  any  degree  of  comfort,  while,  if  the  rooms  be  badly 
illuminated,  the  wall-cases  are  almost  certain  to  be  worse 
lighted  than  any  others.  Glass  cases  standing  away  from 
the  walls  and  fitted  with  shelves  which  range  from  the 
height  of  an  ordinary  table  up  to  about  five  or  six  feet,  and 
so  arranged  that  the  higher  ones  gradually  recede  more  and 
more  from  the  glass,  are  very  good  for  purposes  of  study. 


44  The  Rudiments  of  Petrology. 

Under  any  circumstances  the  receptacles  for  the  specimens, 
whether  cases  or  drawers,  should  be  well  fitted  so  as  to  ex- 
clude dust  as  much  as  possible.  For  eruptive  rocks  to  be 
properly  displayed  in  museums  they  require  quite  as  good 
or  even  better  illumination  than  minerals.  This  is  also 
desirable  for  sedimentary  rocks,  but  is  of  less  importance  as 
a  rule. 


CHAPTER  VI. 

PRELIMINARY    EXAMINATION    OF    ROCKS. 

FOR  the  more  general  and  preliminary  examination  of  rocks 
the  following  implements  will,  if  judiciously  used  (their  use 
being  backed  by  a  moderate  knowledge  of  mineralogy),  be 
found  sufficient  for  simple  investigations.  A  stout-bladed 
penknife  for  testing  hardness,  &c.  Small  fragments  of  the 
minerals  which  constitute  Mohs'  Scale  of  Hardness.  The 
diamond  (No.  10  in  this  scale)  may  be  omitted,  as  rock- 
forming  minerals  seldom  have  a  hardness  exceeding  8.  A 
pocket  magnifier,  one  of  the  ordinary  pattern,  having  two  or 
three  lenses,  will  suffice  for  most  purposes,  but  a  good  Cod- 
dington  lens  is  also  useful  at  times.  There  is,  however,  a 
disadvantage  attending  the  use  of  these  lenses  when  they 
are  applied  to  the  examination  of  rocks.  This  lies  in  the 
difficulty  experienced  by  the  observer  when  he  attempts  to 
examine  the  streak  of  minerals  under  the  lens,  especially 
when  the  minerals  occur  in  very  minute  crystals  or  patches, 
as  it  is  scarcely  possible  to  hold  a  specimen,  with  a  lens 
over  it  in  focus,  in  one  hand,  and  to  work  with  a  knife  in 
the  other.  Laying  the  specimen  on  a  table,  and  using  a  lens 
in  one  hand  and  a  knife  in  the  other,  is  a  most  unsatisfactory 
process,  while  the  use  of  a  lens  fixed  on  an  adjusting  stand 
is  scarcely  better.  To  obviate  this  difficulty  the  author  has 
devised  a  small  lens  with  a  clip,  which  can  be  worn  on  the 
nose  like  an  eye-glass,  and  both  hands  are  then  at  liberty — 


/        , 

Preliminary  Examination  of<R.pcks.       /^4$ 

f  r    .  /  •  *  / 

the  one  to  hold  the  specimen  firmly,  the  other  to  use  the 
knife  or  graver.  This  clip-lens  is,  moreo/e^y-better  tfran  a 
watch-maker's  eye-glass,  because  it  entails  no  rriii^cular  effort 
to  keep  it  in  place.  It  is  better  to  have  the  lens  ^oimted  t  j 
in  a  horn  than  in  a  metal  rim,  as  it  is  less  heavy,  and'  <k)/ise- 
quently  less  liable  to  be  accidentally  shifted  or  displaced  by 
the  inclination  of  the  head.1  A  bottle  of  hydrochloric  acid 
with  a  glass  rod  attached  to  the  stopper  is  useful  in  testing 
for  carbonates.  A  small  rough  unglazed  piece  of  porcelain 
may  be  advantageously  employed  for  determining  the  colour 
of  the  streak  afforded  by  minerals,  but  the  same  end  may  be 
attained  by  scraping  some  of  the  powder  on  a  piece  of  white 
paper,  and  rubbing  and  griming  this  powder  on  to  the  paper 
with  the  side  of  a  knife  blade.  The  blade  of  the  knife  may 
be  magnetised,  and  it  then  answers  well  enough  for  separat- 
ing any  substances  attractable  by  a  magnet  from  pulverised 
fragments  of  rock.  In  most  cases,  however,  it  is  desirable 
to  have  a  freely  suspended  or  supported  magnetic  needle, 
since  by  its  use  in  the  examination  of  rocks  containing  mag- 
netite, &c.,  repulsion  as  well  as  attraction  may  sometimes  be 
observed.  The  operator  may  also,  by  holding  a  sheet  of 
paper  in  front  of  the  mouth  and  nose,  prevent  any  undue 
disturbance  of  the  needle  by  the  breath.  This  is  especially 
needful  in  examining  rocks  in  which  magnetite,  &c.,  exists 
in  only  very  small  proportion.  A  small  dressing-hammer, 
with  a  head  about  two  and  a  half  inches  long,  one  end  with 
a  face  about  half  an  inch  square  and  the  other  end  chisel- 
shaped,  and  a  little  block  of  steel  about  two  inches  square 
and  half  an  inch  thick,  and  with  one  side  polished,  to  serve  as 
an  anvil,  will,  together  with  a  blowpipe,  a  small  blowpipe 
lamp,  some  platinum  wire,  some  pieces  of  well-burnt  char- 
coal and  a  few  fluxes  and  other  blowpipe  reagents,  complete 
the  list  of  apparatus  needful  for  rough  work  ;  and  by  the 
skilful  use  of  these  simple  appliances  quite  as  much  know- 

1  These  lenses  are  manufactured  by  Messrs.  Baker  &  Co. ,  244  High 
Holborn. 


46  The  Rudiments  of  Petrology. 

ledge  may  often  be  gained  as  by  the  employment  of  much 
more  elaborate  methods  of  investigation.  It  is,  however, 
most  desirable  that  the  student  should  possess  some  know- 
ledge of  chemistry,  the  more  the  better,  but  for  most  pur- 
poses, in  the  determination  of  rocks,  a  fair  knowledge  of 
blowpipe  analysis  and  some  familiarity  with  the  more  com- 
mon chemical  reactions  in  the  wet  way  will  be  found  suf- 
ficient, especially  when  the  observer  is  cautious  in  forming 
his  conclusions,  and  bears  in  mind  the  old  adage  that  '  a 
little  knowledge  is  a  dangerous  thing.'  Some  acquaintance 
with  the  more  common  rock-forming  minerals  is  also  abso- 
lutely necessary.  There  are  so  many  manuals  and  text- 
books which  treat  of  these  subjects  that  it  is  difficult  to 
recommend  any  particular  work,  but  a  list  of  some  of  the 
most  useful  will  be  found  at  the  beginning  of  this  volume,  and 
it  may  here  be  as  well  to  remark  that  any  attempt  to  study 
eruptive  rocks  without  a  fair  knowledge  of  mineralogy  may 
justly  be  likened  to  an  attempt  to  read  before  the  alphabet 
has  been  learnt.  It  is  also  very  important  that  the  student 
should  have  a  good  knowledge  of  physical  geology,  so  that 
he  may  be  enabled  to  make  correct  notes  on  the  mode  of 
occurrence  of  rocks  and  form  sound  deductions  from  his 
observations.  In  this  part  of  the  subject,  reading,  unless 
supplemented  by  some  training  in  field  work,  will  be  found 
to  be  of  comparatively  little  use.  For  those,  however,  who 
may  not  have  had  opportunities  of  getting  any  such  training, 
the  remarks  embodied  in  the  foregoing  Chapters  III.  and 
IV.,  if  carefully  studied,  may  prove  useful. 


CHAPTER  VII. 

THE   MICROSCOPE   AND    ITS    ACCESSORIES. 

IN  describing  the  microscopes  suitable  for  examining  thin 
sections  of  minerals  and  rocks  it  will  perhaps  be  best,  in  the. 


Microscopes.  47 

interest  of  students  generally,  to  begin  with  a  description 
of  the  chief  points  to  be  attended  to  in  the  selection  of 
microscopes  of  the  ordinary  patterns  manufactured  and  sold 
in  this  country — instruments  constructed  to  meet  the  require- 
ments cf  the  physiologist,  the  general  microscopist,  and  the 
dabbler  in  science.  Unfortunately  no  microscopes  are  yet 
manufactured  in  this  country  for  the  special  study  of  micro- 
petrology,  but  a  good  instrument  of  the  ordinary  type  will 
be  found  to  answer  the  purpose  very  well  if  a  few  compa- 
ratively inexpensive  alterations  be  made  in  it  It  is  a 
somewhat  difficult  matter  to  recommend  the  microscopes 
of  any  particular  maker.  Those  by  Ross,  Powell  and  Leland, 
Beck,  Baker,  Murray  and  Heath,  Collins,  Browning,  Crouch, 
and  Swift  are  all  good  according  to  the  price.  In  the  in- 
struments by  the  three  first-named  makers  the  optical  and 
mechanical  arrangements  are  carried  to  the  highest  degree  of 
perfection,  while,  in  those  by  the  others,  there  is  a  great  amount 
of  good  workmanship  in  the  more  expensive  microscopes, 
and,  in  some  of  the  cheaper  ones  which  they  supply,  the 
performance  is  very  satisfactory.  There  are  many  other 
makers,  besides  those  just  mentioned,  who  turn  out  gocd 
instruments,  but  as  the  author  has  had  little  or  no  opportu- 
nity of  testing  them,  he  can  only  speak  from  experience,  and, 
having  principally  worked  with  microscopes  made  by  Baker, 
he  thinks  it  only  just  to  say  that  the  performance  of  his  higher 
class  instruments  is  good,  and  that  they  would,  if  supplied 
with  concentrically-rotating,  graduated  stages  and  a  few  other 
fittings,  answer  all  the  ordinary  requirements  of  the  petrolo- 
gist.  Mr.  Watson  of  Pall  Mall  has  lately  been  making  some 
efforts  to  manufacture  microscope  stands,  specially  suited  for 
petrological  work,  at  a  moderate  cost. 

Many  observers  prefer  to  work  with  binocular  micro- 
scopes, but  they  seem  to  offer  few  advantages  and  are  in 
some  respects  inconvenient,  notably  in  the  examination  of 
objects  under  high  powers,  unless  the  prism  be  specially 
constructed  for  their  use  (as  that  by  Powell  and  Leland). 


48  The  Rudiments  of  Petrology. 

They  may  be  very  advantageously  employed  in  the  exami  • 
nation  of  sections  of  vitreous  rocks,  of  detached  crystals,  of 
pulverulent  matter,  as  volcanic  ashes  and  residues  procured 
by  levigation,  but  for  the  ordinary  purposes  of  the  petro- 
logist  a  monocular  instrument  will  in  most  cases  be  found 
to  answer  the  purpose  just  as  well,  if  not  better. 

It  is  essential — 

(1)  That  the  microscope  should  have  a  firm  and  toler- 

ably heavy  stand,  and  that  the  whole  instrument 
should  be  free  from  vibration  when  the  observer 
is  focussing,  or  when  he  is  moving  the  object. 

(2)  That  the  objectives  should  give  a  flat  field,  that  they 

should  be  achromatic,  and  possess  good  definition 
and  penetration.  Better  penetration  is  usually 
procured  with  objectives  which  have  not  a  very 
wide  angular  aperture. 

(3)  That  the  stage  be  so  arranged  as  to  admit  of  the 

object  being  moved  about  an  inch  both  from  back 
to  front  and  from  side  to  side,  and  it  is  also  essen- 
tial that  there  should  be  an  arrangement  for  rota- 
ting the  object.  If  this  rotation  be  concentric, 
so  much  the  better.  The  stage  should  also  be 
graduated  so  that  the  amount  of  rotation  can  be 
accurately  measured. 

(4)  That  the  instrument  be  fitted  with  a  polarising  appa- 

ratus. The  polariser  should  revolve  with  perfect 
freedom,  and  be  so  arranged  that  it  can  be  removed, 
or,  still  better,  displaced  by  turning  on  a  hinge  or 
pivot,  so  as  to  afford  the  means  of  instantly  changing 
from  polarised  to  ordinary  illumination.  The  ana- 
lyser should  be  fitted  in  a  cap  to  slide  easily  over 
the  eye-piece,  so  that  it  can  be  instantly  removed, 
and  there  should  be  an  arrangement  so  that  the 
analyser  and  polariser  can  be  accurately  set  with 
the  longer  diameters  at  right  angles,  or  it  may  be 


Microscopic  Apparatus.  49 

fixed  in  a  sliding  box  or  block  just  above  the 
objective,  as  in  the  microscopes  manufactured  by 
Collins  and  one  or  two  other  makers.1  On  no 
account  should  the  analyser  screw  into  the  top  of 
the  objective,  as  such  an  arrangement  entails  great 
trouble  and  loss  of  time. 

(5)  That  the  eye-piece  of  lowest  power  should  be  fitted 

with  crossed  cobwebs  for  centering  and  for  gonio- 
metric  measurements,  &c.,  and  with  either  a  Beale's 
reflector,  or,  better,  with  a  camera  lucida  (Wol- 
laston's  prism)  for  drawing  objects.  If  the  latter, 
care  should  be  taken  to  see  that  it  performs  well, 
and  that  the  whole  field  is  properly  illuminated. 
The  drawing  made  by  the  first  instrument  is  re- 
versed ;  by  the  Wollaston's  prism  it  is  represented 
in  its  proper  position. 

(6)  That  the  instrument  be  provided  with  a  bull's-eye 

condenser,  or  with  a  silvered  side-reflector,  or  a 
parabolic  speculum,  such  as  that  devised  by  Mr. 
Sorby,  and  manufactured  by  Smith  and  Beck. 

(7)  That  a  stop  be  placed  so  that  the  body  of  the  instru- 

ment can  be  kept  in  a  horizontal  position  when 
drawings  are  being  made  with  the  camera. 

The  most  generally  useful  objectives  are  the  two-inch, 
the  inch,  and  the  half- inch,  but  higher  powers  are  sometimes 
required.  The  objectives  by  some  of  the  English  makers 
are  very  perfect  in  their  performance,  but  some  of  the  Con- 
tinental ones  are  exceedingly  good,  especially  those  by 
Hartnack  and  Gundlach,  and  they  are  less  expensive  than 
those  made  by  many  of  the  English  opticians.  A  good 
objective  should  be  perfectly  achromatic  (i.e.  no  fringes  of 
colour  should  be  visible  around  the  object).  It  ought  also 

1  A  useful  form  of  fitting  for  analyser  and  polariser,  adapted  for  use 
with  any  microscope,  has  lately  been  devised  by  Messrs.  Murray  & 
Heath,  of  Jermyn  Street. 


50  The  Rudiments  of  Petrology. 

to  afford  a  flat  field  (i.e.  when  the  object  is  moved  from  one 
part  of  the  field  to  another  it  should  appear  well  and  sharply 
defined,  the  change  in  its  situation  not  necessitating  any 
alteration  of  focus). 

Mechanical  stages,  such  as  those  moved  by  racks  and 
pinions,  screws,  &c.,  are  not  by  any  means  necessary, 
although  they  are  very  convenient,  especially  when  working 
with  high  powers.  A  well-fitted  sliding  object-carrier  will  be 
found  to  answer  very  well  when  the  hands  have  had  a  little 
practice  in  slowly  moving  the  object  from  back  to  front  of 
the  stage,  while  properly  educated  fingers  are  capable  of 
giving  a  very  steady  transverse  motion  to  the  object.  Spring 
clips,  as  usually  fitted  to  the  "stages  of  the  cheaper  micro- 
scopes, have  both  advantages  and  disadvantages.  Unless 
some  kind  of  clip  be  used  the  object  topples  over  when  the 
microscope  is  placed  in  a  horizontal  position  for  drawing 
with  the  camera,  or  when  the  stage  is  rotated,  while  the 
ordinary  little  spring  clips  are  nuisances  when  it  becomes 
needful  to  examine  sections  which  are  in  course  of  prepa- 
ration, as  they  cannot  accommodate  thick  slabs  of  plate  glass, 
such  as  those  to  which  the  pieces  of  stone  are  cemented 
when  being  ground  down.  Sub-stages  or  supplementary 
stages  (for  carrying  the  polariser,  spot  lens,  &c.),  are  of  little 
or  no  use  to  the  petrologist.  They  simply  encumber  the 
microscope  with  troublesome  adjustments,  which  are  seldom 
or  never  used,  and  which  only  get  in  the  way.  Moreover, 
when  the  polariser  is  fitted  to  a  sub-stage  a  screen  of  some 
kind  ought  to  be  used  to  prevent  the  passage  of  extraneous 
light  between  the  top  of  the  polarising  prism  and  the  bottom 
of  the  stage,  otherwise  erroneous  conclusions  may  be  arrived 
at  with  regard  to  singly  refracting  substances.  It  is  far 
better  that  the  polariser  should  be  directly  attached  to  the 
under  surface  of  the  stage. 

An  achromatic  condenser  is  occasionally  useful  to  the 
petrologist  when  examining  feebly  translucent  sections.  If 
the  stage  of  the  microscope  be  moved  with  rackwork  a 
Maltwood's  finder  will  be  found  convenient. 


Finders.  5 1 

This  consists  of  a  series  of  minute  squares  photographed 
upon  a  glass  slide,  each  square  bearing  two  numbers  (fig.  19). 
This  finder  affords  a  ready  means  of  FIG 

registering  the  exact  position  of  any  mi- 
nute object  in  a  preparation.  Unless  a 
finder  be  employed,  much  time  is  often 
lost  in  the  endeavour  to  re-discover  any 
very  small  object  or  any  particular  portion 
of  a  section  which  has  been  previously 
observed.  It  is  used  in  the  following  manner: — 

A  small  peg  or  stop  is  screwed  into  the  object-carrier  so 
that  one  end  of  an  ordinary  slide  may  abut  against  it. 

When  any  particular  object  or  spot  is  noticed,  and  it  is 
desirable  to  record  its  position,  the  object  is  removed,  with- 
out touching  any  of  the  mechanical  adjustments  of  the  stage. 
The  finder  is  then  placed  on  the  stage,  with  one  end  closely 
in  contact  with  the  stop  against  which  the  end  of  the  object- 
slide  previously  rested;  the  instrument  is  re-focussed,  if 
necessary,  in  order  to  read  the  numbers  in  the  centre  of  the 
field.  These  numbers  may  then  be  recorded  either  on  the 
label  of  the  slide  itself  or  in  a  note-book.  When  it  is  again 
requisite  to  find  the  object,  the  finder  is  put  on  the  stage  as 
before,  and  is  moved  about  until  the  recorded  numbers  are  in 
the  centre  of  the  field.  It  is  then  removed,  the  preparation 
is  put  in  its  place,  and  the  required  object  will  be  found 
to  occupy  the  place  of  the  recorded  numbers. 

Another  finder,  applicable  to  microscopes  which  have 
not  mechanical  stages,  consists  in  a  metallic  arm,  hinged  or 
pivoted  to  a  fixed  portion  of  the  microscope  above  the 
stage.  The  arm  is  curved,  and  terminates  in  a  point.  When 
it  is  needful  to  register  the  exact  position  of  some  object  in 
the  field  of  view,  the  point  of  the  arm  is  daubed  with  ink, 
and  it  is  then  brought  down  upon  a  paper  label  pasted  on, 
one  end  of  the  slide  which  carries  the  preparation.  There 
it  imprints  a  dot.  By  again  bringing  this  dot  under  the 
point  of  the  arm  the  desired  object  can  at  once  be  brought 

E  2 


The  Rudiments  of  Petrology. 


FIG.  20. 


into  the  field.  To  those  who  devote  much  time  to  making 
microscopic  drawings  it  will  be  useful  to  have  a  small  movable 
needle  or  indicator  N  (fig.  20),  mov- 
able by  a  milled  head  H,  fitted  into 
one  of  the  eye-pieces.  By  this  means 
it  is  easy  to  find  the  spot  at  which 
the  draughtsman  was  last  looking. 

Brook's  double  nose-pieces,  or 
the  triple  or  quadruple  nose-pieces 
devised  by  various  makers,  are  con- 
venient, inasmuch  as  they  facilitate 
the  rapid  interchange  of  objectives 
of  varying  power.  Their  workman- 
ship is,  however,  seldom  sufficiently 
good  to  render  their  use  desirable 
with  high  powers,  and  their  constant 
employment  does  not  tend  to  im- 
•prove  the  performance  of  the  fine 

adjustment,  with  the  tube  of  which  they  are  connected. 
Those  nose-pieces  which  are  turned  up,  so  that  the  objec- 
tives do  not  stand  parallel  when  screwed  on,  are  to  be  pre- 
ferred, .as  they  permit  the  free  use  of  objectives  of  varying 
length  and  of  different  foci, -without  the  risk  of  jamming  an 
objective  against  the  stage,  and  so  deranging  the  mechanism 
of  the  nose7piece  and  fine  adjustment,  together  with  other 
possible  damage  to  the  instrument. 

In  selecting  microscopes,  those  on  the  new  Jackson 
model  are  greatly  to  be  preferred  for  rigidity,  steadiness,  and 
accuracy  of  centering. 

The  use  of  very  deep  eye-pieces  should  be  avoided  ;  an 
A  and  B  or  a  No.  i  and  No.  2  eye-piece  will,  as  a  rule, 
be  all  that  are  requisite.  Deep  eye-pieces  entail  feeble 
illumination,  and  augment  any  error  which  the  objective 
may  give  rise  to. 

Frog-plates,  live-boxes,  compressoria,  and  other  like  ap- 
paratus, usually  supplied  with  microscopes,  are  of  no  use 
whatever  to  the  petrologist. 


Goniometers.  53 

It  is  occasionally  very  important  to  be  able  to  determine 
with  some  degree  of  precision  the  angles  presented  by  sec- 
tions of  crystals  as  they  occur  in  sections  of  rocks,  or  the 
angles  of  minute  individual  and  unattached  crystals.  In  the 
latter  case  it  is  essential  that  the  crystal  should  be  so  arranged 
that  the  faces  whose  mutual  inclinations  have  to  be  measured 
are  situated  in  planes  parallel  to  the  axis  of  vision.  In  the 
sections  of  crystals  which  occur  in  thin  slices  of  rocks,  it  is 
frequently  a  matter  of  considerable  difficulty  to  ascertain  in 
what  direction  the  section  has  been  cut ;  and  such  sections 
are  not  very  often  cut  in  directions  precisely  at  right  angles 
to,  or  parallel  to,  the  principal  crystallographic  axis,  or  to 
any  one  of  the  lateral  axes ;  consequently  measurements  of 
the  angles  offered  by  such  sections  are,  as  a  rule,  only 
approximate.  Although,  therefore,  a  very  exact  goniometer 
is  desirable,  a  tolerably  good  one  will  usually  suffice.  The 
simplest  method  of  measuring  angles  under  the  microscope 
is  to  place  the  microscope  in  a  horizontal  position,  and  by 
means  of  the  camera  to  draw  the  outlines  of  the  two  faces  to 
be  measured  on  a  good-sized  sheet  of  paper ;  these  lines 
should  then  be  prolonged,  and  the  angle  at  which  they 
lie  to  one  another  can  easily  be  measured  with  a  common 
protractor.  By  this  means,  however,  measurements  cannot 
be  got  to  within  less  than  30'  or  at  best  15  .  Still,  if  the 
section  be  not  cut  in  exactly  the  proper  plane,  the  results 
given  by  a  more  precise  instrument  would  not  be  of  any 
greater  practical  value.  A  very  good  goniometer,  that  known 
as  Schmidt's,  a  form  of  which  is  manufactured  by  Ross  and 
Powell  and  Leland,  consists  of  a  positive  eye -piece  in  which 
a  cobweb  is  placed ;  around  this  eye-piece  there  is  a  gra- 
duated brass  circle  about  four  inches  in  diameter,  and  an 
arm  carrying  a  vernier  and  set  screw  is  attached  directly  to 
the  eye-piece,  so  that  when  the  arm  is  moved  the  eye- 
piece itself  turns  together  with  the  cobweb.  In  using  this 
instrument,  the  cobweb  is  aligned  on  one  of  the  faces  of  the 
crystal,  and  the  movable  index  is  brought  to  O  on  the  circle. 


54  The  Rudiments  of  Petrology. 

The  arm  is  then  turned  until  the  cobweb  is  aligned  on  the 
adjacent  face,  the  set-screw  is  turned  to  clamp  the  arm,  and 
the  number  of  degrees,  minutes,  &c.5  through  which  the  arm 
has  travelled  is  read  off  by  means  of  the  vernier,  over  which 
a  small  magnifier  is  usually  placed.  When  the  angles  of 
crystals  occurring  in  sections  of  rock  which  are  not  very 
translucent  have  to  be  measured  by  this  instrument,  diffi- 
culty is  often  experienced  in  seeing  the  cobweb  distinctly, 
and  this  is  one  of  the  most  serious  drawbacks  to  the  use  of 
this  kind  of  goniometer  for  petrological  purposes.  Its  utility 
would  probably  be  increased  if  one  half  of  the  field  were 
obscured  by  the  insertion  of  a  blackened  semicircle  of  metal 
within  the  focus  of  the  eye-piece  instead  of  the  cobweb. 
Measurements  of  the  angles  of  crystals  may  also  be  made  by 
the  use  of  a  positive  eye-piece  carrying  a  cobweb  and  a 
concentrically  revolving  stage,  if  the  stage  be  graduated  on 
the  margin ;  but  as  such  stages  are  only  furnished  witb  first- 
class  instruments,  the  majority  of  students  are  not  likely  to 
avail  themselves  of  this  method. 

A  microscope  specially  constructed  for  mineralogical  and 
petrological  purposes  has  recently  been  devised  by  Prof. 
Rosenbusch,  of  Strassburg.  The  chief  advantages  of  this 
instrument  consist — 

(1)  In  the  facilities  for  turning  an  object  in   its   own 

plane  between  fixed  crossed  Nicols,  the  rotation 
being  concentric  with  the  axis  of  vision. 

(2)  In  the  ability  to  read  off  accurately  the  angle  through 

which  the  object  may  be  turned  in  a  horizontal 
plane  by  means  of  the  graduation  around  the  cir- 
cular stage. 

(3)  In  the  facility  with  which  the  polariser  and  analyser 

can  be  displaced  and  replaced,  and  the  means  by 
which  the  exact  position  of  the  principal  sections 
of  the  polariser  and  analyser  can  be  noted. 

(4)  Where  the  total  extinction   of  light  by  means   of 

crossed  Nicols  interferes  with  the  researches  on 


Prof.  Rosenbusctts  Microscope. 


55 


FIG.  21. 


any  mineral,  means  are  provided  for  facilitating 
observation  under  such  circumstances. 
The  peculiarities  in  the  construction  of  this  microscope 
consist  in  trie  tube  which  carries  the  eye-piece  and  objective 
m  n  (fig.  21),  being,  as  it  were,  suspended  within  an  outer 
tube  op,  its  only  attachment  being  at  the  top  at  b  c.  A 
block,  r,  is  fixed  between  the 
inner  and  outer  tubes  to  prevent 
any  rotation  during  focal  adjust- 
ment. This  coarse  adjustment  is 
effected  by  hand,  as  in  some  of 
the  cheaper  English  microscopes, 
the  thumb  and  forefinger  sliding 
the  inner  tube  up  and  down  by 
pressure  on  the  disc  d >,  other 
fingers  being  applied  to  the  top, 
b  c,  of  the  fixed  tube.  The  fine 
adjustment  consists  of  a  micro- 
meter screw,  shown  at  a.  The 
unattached  portion  of  the  inner 
tube  is  steadied  in  the  outer  one 
by  means  of  a  spring  and  three 
little  screws  set  horizontally  and 
capped  with  scraps  of  parchment. 
The  arm  of  the  microscope  carries 
two  screws  with  milled  heads, 
one  of  which  is  shown  at  h. 
These  are  set  at  right  angles  to  p^ 

one  another,  and  serve  to  centre     ' ' 

the  tube  in  a  manner  presently  to  be  described.  Each 
eye-piece  carries  two  cobwebs  within  it,  which  intersect  at 
right  angles  in  the  centre  of  the  field.  To  the  outside  tube 
of  each  eye-piece  a  small  peg  is  fixed,  which  slides  into  a 
corresponding  slot  in  the  top  of  the  inner  movable  tube  of 
the  microscope.  This  arrangement  prevents  any  rotation  of 
the  eye-pieces,  and  so  keeps  the  cobwebs  in  a  fixed  posi- 


5  6  Tlie  Rudiments  of  Petrology. 

tion.  An  analyser,/,  fitting  in  a  brass  cap,  slides  easily  over 
the  top  of  the  eye-piece.  The  bottom  of  the  cap  is  sur- 
rounded by  a  bevelled  flange,  g,  which  is  graduated  to  5°. 
An  index  mark  on  the  plate,  a  e,  serves  to  record  the 
angle  through  which  the  prism  may  be  rotated.  The  stage, 
/,  of  the  microscope  has  a  circular  form,  and  a  circular 
plate  or  object-table,  /£,  is  arranged  so  as  to  revolve  horizon- 
tally on  it.  This  table  is  graduated  on  its  margin,  and 
an  index  to  record  the  amount  of  the  revolution  which 
may  be  imparted  to  it  is  attached  to  the  front  of  the  fixed 
stage  at  t.  Beneath  the  stage  is  set  an  easily  displaceable 
polariser,  consisting  of  a  Nicol's  prism,  which  revolves 
within  its  external  tube  by  means  of  the  disc,  v,  which  is 
graduated  to  10°,  and  has  its  index  marked  on  the  fixed 
outer  tube,  z.  This  polariser  does  not  turn  when  the  object- 
table  is  rotated,  but  remains  unaltered  in  position.  A  plate 
of  quartz,  375  millimeters  thick  and  mounted  in  a  little  brass 
fitting,  is  shown  at  q.  It  slides  into  a  corresponding  slot, 
situated  close  to  the  lower  end  of  the  inner  microscope  tube 
and  above  the  objective,  which  is  omitted  in  the  figure.  A 
small  plate  of  calcspar  for  making  stauroscopic  measure- 
ments is  also  supplied  with  the  microscope,  together  with  a 
brass  ring  for  fixing  it  above  the  eye-piece  and  beneath  the 
analyser. 

The  following  directions  for  using  this  microscope  are 
extracted  from  Professor  Rosenbusch's  paper.1 

If  any  particular  spot  in  an  object,  such  as  a  granule  of 
magnetite,  be  brought  exactly  under  the  point  of  intersection 
of  the  eye-piece-cobwebs,  i.e.  in  the  middle  of  the  field  of 
vision,  and  the  object-table  be  then  turned  in  its  horizontal 
plane,  the  inner  tube  of  the  microscope  will  be  found  to 
hang  neither  vertically  nor  concentrically  without  the  inter- 
vention of  the  centering  screws,  while  the  spot  under 
observation  will  not  remain  in  the  centre  of  the  field,  and 

1  Ein  nenes  Mikroskop  fur  mineralogiscJie  und  petrographische 
Untersuchungen.  H.  Rosenbusch.  Neues  Jahrbuch  fiir  Mineralogie. 
1876. 


Prof.  Roscnbusctis  Microscope. 


57 


FIG.  22. 


under  the  point  of  intersection  of  the  cross-bars  a  a  and  /3  /3, 

(fig.  22),  but  will  describe  an  eccentric  circle  somewhat  in 

the  manner  shown  in  fig.  22.     The  tube  of  the  microscope 

must  therefore  be  placed  vertically ;  in   other  words,  the 

instrument  must  be  centered  by 

means    of  the    two    centering 

screws,  one  of  which  is  shown 

at  h  in  fig.  21.     It  will  be  seen 

by  fig.  22  that  the  optical  axis  of 

the  microscope  is  at  o,  and  not 

at  <?,  and,  in  order  to  get  proper 

centricity  in  the  movement,  the  <j 

spot  o f  should  be  made  to  coin- 
cide with  o.  To  effect  this  the 
end  of  the  tube  must  be  moved 
in  the  direction  of  o,  o.  By  means  of  one  centering  screw 
it  will  be  driven  in  the  direction  <>t  v,  and  by  the  other  in  the 
direction  «•  fj.  When  these  adjustments  have  been  properly 
made,  the  spot  should  be  brought  exactly  under  the  inter- 
section of  the  cross-bars  in  the  eye-piece,  and  should  remain 
there  during  the  revolution  of  the  object-table. 

When  this  operation  is  once  properly  performed,  any 
other  spot  or  part  of  an  object  which  may  be  brought  into 
the  field  will,  upon  rotation  of  the  object-table,  be  found  to 
revolve  concentrically,  so  long  as  the  same  eye-piece  and 
the  same  objective  are  used,  but  if  one  or  other  of  them 
be  changed,  it  will  usually  be  necessary  to  re-centre  the 
instrument.  This  will,  however,  generally  entail  only  a 
slight  alteration  of  the  centering  screws.  The  movement 
imparted  to  the  microscope  tube  by  these  screws  tends  to 
throw  the  analyser  slightly  out  of  position  with  regard  to 
the  polariser,  but  the  inventor  finds  that  this  produces 
scarcely  any  appreciable  error.  In  testing  the  pleo- 
chroism  of  a  mineral,  the  object-table  bearing  the  sec- 
tion may  be  revolved  above  the  fixed  polariser,  or  the 
polariser  may  be  turned  beneath  the  stage,  the  graduations 


58  The  Rudiments  of  Petrology. 

affording   facilities   for    determining    the   position    of    the 
pleochroitic  maxima. 

The  principal  directions  of  vibration  in  a  mineral  sec- 
tion may  be  determined  by  inserting  it  in  the  maximum 
extinction  of  light  between  crossed  Nicols,  but  since  the  eye 
is  incapable,  under  these  circumstances,  of  appreciating  in 
certain  cases  very  slight  differences  in  the  transmission  of 
light  by  depolarisation,  a  calcspar  plate  is  inserted  in  the 
stauroscope  between  the  analyser  and  the  section  of  the 
mineral  under  examination.  The  interference  figure  of'  the 
calcspar  appears  distorted,  until  a  direction  of  principal 
vibration  in  the  section  coincides  with  that  of  the  polariser. 
In  stauroscopic  measurements  very  precise  results  may  be 
arrived  at  by  the  employment,  not  of  ordinary  white  light, 
but  of  monochromatic  light  derived  from  a  coloured  gas 
flame  ;  this  method,  however,  although  useful  when  an 
ordinary  stauroscope  is  used,  is  inapplicable  to  microscopic 
research.  In  microscopic  examinations  a  plate  of  quartz 
375  millimeters  thick  is  used  instead  of  a  calcspar  plate  ; 
where  this  is  employed  a  monochromatic  field  is  procured. 
When  the  principal  direction  of  vibration  in  the  analyser  is 
turned  at  a  different  angle  to  that  of  the  polariser,  the  field 
will  become  changed  to  various  colours,  where  doubly 
refracting  bodies  are  situated  in  the  field  of  view,  and  their 
principal  directions  of  vibration  do  not  coincide  with  that 
of  the  polariser.  By  turning  the  object-table  until  such 
coincidence  is  arrived  at,  a  purely  monochromatic  field 
will  be  produced ;  very  slight  movement  of  the  object 
will  again  suffice  to  destroy  the  monochrome.  The  employ- 
ment of  such  a  quartz-plate  is  most  useful  when  very  feebly 
double-refracting  media  are  being  examined,  and  also  for 
detecting  isotropic  particles  in  rocks  with  admixtures  of 
amorphous  paste  of  a  doubtful  character.  In  addition  to 
the  apparatus  here  described  this  microscope  as  manufac- 
tured by  Fuess  of  Berlin  (Alte  Jacob  Strasse,  108)  is 
supplied  with  three  eye -pieces  and  three  objectives  of  Hart- 
nack's  make,  which  give  a  range  of  amplification  varying  from 


Preparation  of  Sections.  59 

about  90  to  1,150  diameters,  and  by  the  use  of  the  different 
eye-pieces  affording  a  series  of  9  different  powers.  An 
eye-piece  micrometer  and  an  apparatus  for  heating  objects 
under  examination,  and  recording  the  temperature  by  means 
of  a  thermometer,  are  also  supplied  with  this  instrument. 
Some  useful  modifications  of  Prof.  Rosenbusch's  microscope 
have  been  made  by  Prof.  Renard  and  described  by  him  in  the 
'  Bulletins  de  la  Societe  beige  de  Microscopic/  tome  iv.  1877-78. 
In  the  'Neues  Jahrbuch  fur  Mineralogie,  &c.,'  1878^.377, 
Professor  A.  von  Lasaulx  has  described  methods  of  con- 
verting ordinary  microscopes  so  that  they  can  be  employed 
for  the  examination  of  minerals  in  convergent  polarised 
light.  Another  paper  by  the  same  author,  op.  cit.  p.  509, 
written  a  couple  of  months  later,  describes  the  construction 
of  a  polariscope,  suitable  for  purposes  of  demonstration, 
which  consists  in  part  of  the  tube  and  Nicol's  prisms  of  an 
ordinary  Hartnack's  microscope.  A  microscope  devised 
for  mineralogical  and  chemical  purposes  was  devised  some 
years  since  by  Dr.  Leeson.  It  was  manufactured  and 
improved  by  Mr.  Highley,  by  whom  it  is  described  in  the 
1  Quarterly  Journal  of  Microscopical  Science,'  p.  281.  Another 
microscope,  specially  constructed  for  the  examination  of 
substances  in  hot  acid  solutions  and  corrosive  fluids,  has 
been  devised  by  Dr.  Lawrence  Smith.  In  this  instrument 
the  stage  is  placed  above  the  objective,  and  the  object  is 
viewed  from  the  under  surface  of  the  slide.1 


CHAPTER   VIII. 

METHOD     OF     PREPARING    MINERALS    AND     ROCKS     FOR 
MICROSCOPIC    EXAMINATION. 

THE  preparation  of  thin  sections  of  minerals  and  rocks  for 
microscopic  examination,  although  effected  by  simple  means, 

1  American  Journal  of  Science,  2nd  series,  vol.  xiv.  1852.  The 
instrument  is  figured  in  How  to  Work  -with  the  Microscope,  by  Dr. 
Lionel  S.  Beale. 


60  The  Rudiments  of  Petrology. 

presents  numerous  difficulties  to  those  who  have  had  no 
previous  experience  in  work  of  this  kind.  The  object  of 
the  present  chapter  is  to  supply  plain  instructions  concerning 
the  needful  appliances  and  the  methods  of  manipulation 
by  which  such  sections  may  be  successfully  made. 

It  is  true  that  sections  may  be  prepared  by  lapidaries,1 
and  that  the  student  is  thus  spared  considerable  labour  and 
loss  of  time  ;  but  he  will  find,  at  all  events  in  the  earlier 
stages  of  his  work,  that  there  are  certain  advantages  which 
he  will  derive  from  the  preparation  of  his  own  sections. 
These  advantages  consist  mainly  in  the  facilities  which  he 
will  have  for  testing  the  hardness  of  minerals,  their  deport- 
ment with  chemical  reagents,  and  the  different  appearances 
which  they  present  when  examined  at  intervals  while  the 
process  of  grinding  them  thinner  and  thinner  is  being 
carried  on.  The  apparatus  needful  for  such  work  is  of  a 
very  simple  kind,  but  more  or  less  complex  appliances  for 
cutting  and  grinding  will  be  found  advantageous. 

As  it  is  desirable  to  lessen  the  labour  of  grinding  as 
much  as  possible,  the  first  thing  to  be  done  is  to  procure  a 
thin  chip  or  a  thin  slice  of  the  mineral  or  rock  about  to  be 
examined.     A  square  inch  is  a  convenient  size  for  the  chip 
FIG  2  or  slice,  as  such  a  piece  will  often  undergo 

considerable  diminution  before  it  is  reduced 
to  a  sufficiently  thin  state.  Chips  may  be 
procured  by  using  a  small  hammer,  but  fre- 
quently a  number  of  flakes  have  to  be 
struck  off  before  one  of  suitable  size,  thinness, 
and  flatness  is  got.  When  the  specimen  is 
very  small,  and  difficult  to  hold  in  the  hand 
while  the  hammer  is  used,  a  satisfactory  chip 
may  often  be  procured  by  holding  the  fragment  in  a  suitable 
position  on  the  edge  of  a  cold  chisel  either  let  into  a  block 
of  wood  (fig.  23),  or  screwed  into  a  vice,  but  then  the 

1  Mr.  F.  G.  Cuttell  (52  New  Compton  Street,  Soho)  prepares  ad- 
mirable sections. 


Preparation  of  Sections.  6 1 

operator  must  take  care  of  his  fingers.  In  the  chipping  of 
very  hard  rocks  it  is  also  advisable  to  protect  the  eyes, 
especially  when  the  hammerer  is  not  well  practised  in  stone- 
breaking.  For  this  purpose  a  pair  of  wire-gauze  spectacles 
will  be  found  useful.  When  cleavable  minerals  are  to  be 
dealt  with  it  is  best  to  avail  oneself  of  the  cleavage,  but  also 
to  note  in  which  direction  of  cleavage  the  plate  is  struck  off, 
and,  if  it  be  desirable  to  make  a  section  in  some  other 
plane  than  that  of  cleavage,  a  slitting  or  sawing  process, 
hereafter  to  be  described,  is  the  only  way  in  which  such  a 
section  can  be  procured. 

When  a  suitable  chip  has  been  struck  off  the  specimen, 
the  first  thing  to  be  done  is  to  grind  one  side  of  it  perfectly 
flat.  This  may  be  accomplished  either  by  grinding  it  by 
hand  on  a  flat  cast-iron  plate  with  moderately  fine  emery 
and  water,  or  by  using  a  machine  with  a  revolving  leaden 
lap,  similarly  charged  for  the  purpose.  The  former  method 
is  the  more  tedious,  and,  although  preferred  by  some  people, 
is  far  less  convenient  than  the  latter,  supposing  the  operator 
to  have  a  suitable  machine  at  his  command.  There  are 
various  forms  of  machines  which  have  been  devised  for 
this  purpose,  some  of  them  being  worked  by  a  treadle  and 
others  by  hand  ;  the  latter  are  the  more  portable,  but  the 
former  are  usually  considered  easier  to  work.  Machines 
of  both  kinds  are  manufactured  by  Fiiess,  of  Berlin,  and  other 
makers.  Good  treadle  machines,  devised  by  Mr.  J.  B. 
Jordan,  of  the  '  Mining  Record '  Office,  may  be  procured  from 
Messrs.  Cotton  &  Johnson,  21,  Grafton  Street,  Soho,  and 
will  be  found  to  be  well  suited  for  the  purpose.  These 
machines  are  supplied  with  slitting  discs  for  sawing  off  thin 
slices  of  rocks  or  minerals,  and  with  laps  for  grinding  them 
down  to  the  requisite  degree  of  thinness. 

The  following  details  of  construction,  extracted  from  the 
Journal  of  the  Quekett  Microscopical  Club,  together  with 
the  use  of  the  illustration,  have  been  kindly  furnished  by 
the  inventor  : — '  As  will  be  seen  from  the  diagram  below, 


62 


The  Rudiments  of  Petrology. 


this  machine  consists  of  a  wooden  frame-work,  a  a,  support- 
ing a  crank-axle  and  driving-wheel,  the  latter  being  two  feet 
in  diameter ;  the  top  part  of  this  frame  is  formed  of  two 
cross-pieces,  #',  fixed  about  an  inch  apart,  as  in  the  bed  of 
an  ordinary  turning-lathe  ;  into  the  slot  between  them  is 
placed  a  casting,  B,  carrying  the  bracket  for  the  angle- 
pulleys,  c ;  this  casting  is  bored  to  receive  the  spindle,  D, 

FIG.  24. 


which,  by  means  of  a  treadle,  can  be  made  to  revolve  at 
the  rate  of  400  or  500  revolutions  per  minute ;  it  is  also 
fitted  with  another  spindle,  E,  having  a  metal  plate,  F, 
fixed  on  the  top,  for  carrying  the  small  cup,  H,  to  which  the 
specimen  is  attached  by  means  of  prepared  wax.  This 
method  of  mechanically  applying  the  work  to  the  slicer  is 
far  preferable  to  holding  it  in  the  hand  in  the  ordinary  way ; 


Preparation  of  Sections.  63 

the  requisite  pressure  against  the  cutting  disc  is  regulated 
by  the  weight,  G,  and  the  thickness  of  the  slice  by  the 
thumb  screw,  K,  on  which  the  spindle  rests.  By  this  means, 
it  is  possible  to  cut  thin  and  parallel  slices — the  thinness  of 
course  varying  according  to  the  strength  of  the  rock  which 
is  being  operated  upon.  The  slitting  disc  is  made  of  soft 
iron,  eight  inches  in  diameter,  and  about  ^  of  an  inch  in 
thickness,  and  it  is  fixed  on  the  spindle,  D,  between  two 
brass  plates  or  washers,  four  inches  in  diameter,  by  means 
of  the  nut,  n.  The  lap  or  grinding  disc  is  eight  inches  in 
diameter,  of  lead  or  cast  iron  about  f  of  an  inch  thick  in 
the  centre,  and  having  rounded  edges  arid  slightly  convex 
sides  ;  this  form  facilitates  the  grinding  of  uniform  thinness, 
there  being  always  a  tendency  on  a  flat  surface  (which  soon 
wears  hollow)  for  the  edges  of  the  section  to  grind  away 
before  it  is  sufficiently  thin  towards  the  centre.' 

In  using  such  a  machine  for  slitting  off  slices  the  edge 
of  one  of  the  thin  iron  discs  should  be  charged  with 
diamond  dust.  This  should  be  worked  into  a  paste  on  a 
slab  or  in  a  small  watch-glass  (a  stand  for  which  may  be 
made  with  the  rim  of  a  pill-box  cover),  with  a  little  sweet  oil, 
and  the  mixture  taken  up  in  small  quantities  on  the  end  of 
a  crow-quill  suitably  cut,  and  it  should  then  be  applied 
carefully  to  the  edge  only  of  the  disc,  the  disc  being  slowly 
turned  by  hand  for  a  short  distance,  say  an  inch  or  two, 
and  afterwards  rubbed  in  hard  with  a  short  but  thick  piece 

FIG.  25.  //  ^    I     it    if 


'"'^'T/'Y  )/,  v 

of  glass  cylinder  about  half  an  inch  long,  fitted  to  a  prongdd  >  J 


handle  in  such  a  manner  that  it  acts  as  a  roller  (fig.  25). 
This  process  should  be  continued  until  the  entire  edge  of 
the  disc  has  been  well  charged.  The  piece  of  stone  to  be  slit 


64  The  Rudiments  of  Petrology. 

should  then  be  fixed  firmly  in  the  small  metal  cup  which  is 
afterwards  to  be  clamped  in  the  movable  arm  or  plate  pro- 
vided for  its  reception.  The  chip  or  fragment  of  stone 
may  be  fixed  in  the  cup  by  means  of  Waller's  wax,  otherwise 
known  as  red  cement.  Small  fragments  of  the  cement 
should  be  placed  in  the  cup  and  the  whole  held  over  a 
spirit  lamp  or  a  Bunsen's  gas-burner  until  the  wax  is  fairly 
melted.  The  piece  of  stone,  previously  warmed,  is  then 
pressed  firmly  into  the  wax. 

It  is  well  to  press  the  cement  while  yet  warm  closely 
round  the  fragment,  which  is  best  done  with  a  cold  metal 
point.  It  may  then  be  allowed  to  remain  until  it  is 
quite  cold.  After  fixing  the  cup  into  its  arm  or  plate, 
the  latter  should  be  adjusted  to  the  proper  height  for  the 
disc  to  make  the  first  slice.  Suitable  pressure  should 
Jthen  be  applied  either  by  hand,  by  a  pulley  and  weight,  or 
by  an  elastic  spring,  fitted  by  one  end  to  an  upright  rod  on 
the  table  of  the  machine  and  by  the  other  to  a  stud  fixed  on 
the  carrying  arm.  Under  any  circumstances  it  is  better  to 
assist  the  regulation  of  pressure  by  hand.  It  should  also  be 
observed  that,  in  commencing  the  process  of  slitting,  the 
edge  of  the  disc  should  first  be  brought  into  contact  with  a 
comparatively  flat  or  rounded  surface  of  the  fragment  of 
stone  and  not  with  a  sharp  edge,  as,  in  the  latter  case,  the 
diamond  dust  will  probably  be  stripped  off  during  the  first 
revolution  of  the  disc.  This  is  a  point  to  be  very  carefully 
observed  :  indeed  it  is  better  for  the  first  few  revolutions  to 
apply  the  pressure  entirely  by  hand.  Oil-of- brick,  or  a  mix- 
ture of  soft-soap  and  water,  should  then  be  applied  to  the 
edge  of  the  disc  at  a  spot  just  in  front  of  the  stone,  so  that 
it  may  be  properly  lubricated  before  it  traverses  the  hard 
surface,  and  on  no  account  ought  the  disc  to  become  dry 
while  cutting,  or  the  diamond  edge  will  instantly  be  lost. 
The  application  of  the  lubricant  may  be  made  either  by 
means  of  a  brush  held  in  the  left  hand  or  by  a  dripping 
apparatus,  such  as  a  tin  pot  with  a  very  small  tap  fixed  in 


Preparation  of  Sections.  65 

the  bottom.  The  disc  should  next  be  set  in  motion  and  a 
steady  pressure  and  constant  lubrication  kept  up  until  the 
slice  is  cut  off.  The  carrying  arm  should  then  be  raised, 
say  one-eighth  or  one-sixteenth  of  an  inch,  according  to  the 
tenacity  of  the  mineral  or  rock  which  is  being  dealt  with, 
and  a  like  process  should  be  repeated  until  the  second  cut 
is  finished,  when  the  slice  is  ready  for  grinding.  The  pro- 
cesses connected  with  the  grinding  of  a  sawn-off  slice  and  of 
a  hammer-chipped  fragment  are  identical.  The  leaden  lap 
should  be  substituted  for  the  slitting  disc.  Two  pots  or 
saucers  should  be  at  hand,  the  one  filled  with  moderately 
fine  emery  and  the  other  with  water.  A  house-painter's 
brush  (a  small  sash  tool  as  it  is  technically  termed)  should 
then  be  dipped  in  the  water  and  afterwards  in  the  emery, 
and  the  resulting  paste  smeared  all  over  the  upper  surface 
of  the  leaden  grinding-lap.  The  machine  should  then 
be  set  in  motion  and  the  slice  or  fragment  be  firmly 
pressed  on  the  surface  of  the  lap  by  the  fingers  of  one  or 
of  both  hands,  care  being  taken  to  keep  the  finger-tips 
clear  of  the  revolving  lap.  A  little  practice  will  soon  teach 
the  operator  the  best  way  of  doing  this.  When  a  good, 
even  surface  is  procured  in  this  way,  the  slice  or  chip 
ought  to  be  carefully  washed  and  wiped  to  free  it  from 
all  adhering  particles  of  coarse  emery,  and  then  the  some- 
what rough  surface  should  be  rendered  as  smooth  as 
possible  by  grinding  the  fragment  by  hand  on  a  flat  brass 
slab,  or  on  a  slab  of  thick  plate  glass,  about  six  or  seven 
inches  by  four  or  five  inches  in  diameter,  smeared  with  the 
finest  flour-emery  and  water.  The  motion  of  the  hand  in 
grinding  should  be  a  circular  one,  and  it  should  be  carried 
systematically  all  over  the  plate,  so  that  the  latter  may  not 
become  unequally  worn.  When  a  perfectly  smooth  surface 
is  procured  the  process  must  be  stopped  and  the  fragment 
again  thoroughly  washed  and  cleansed  from  all  adhering 
emery.  The  next  process  consists  in  cementing  the  smooth 

F 


66 


The  Rudiments  of  Petrology. 


FIG.  26. 


surface  of  the  stone  to  a  small  slab  of  plate  glass  about 
two  inches  square  and  about  a  quarter  of  an  inch  or  more  in 
thickness,  the  edges  being  previously 
ground  roughly  on  the  lap  to  avoid 
the  risk  of  cutting  the  fingers,  in  case 
it  should  slip  when  pressed  on  the 
revolving  disc.  One  of  these  small 
glass  slabs  should  be  placed  upon 
an  iron,  brass,  or  copper  plate,  sup- 
ported either  on  a  tripod  or  by  other 
means,  over  a  Bunsen's  gas  jet  (fig. 
26)  or  a  spirit  lamp,1  and  a  few  scraps  of  the  oldest  and  driest 
Canada  balsam  which  can  be  procured  should  be  laid  upon 

the  top  of  the  glass,  the 
piece  of  stone  to  be 
cemented  also  being 
laid  on  the  iron  plate 
(but  not  on  the  glass 
slab)  with  its  smoothly 

ground  surface  uppermost.  The  jet  or  lamp  should  now  be 
lighted  and  the  gradual  liquefaction  of  the  balsam  carefully 
watched.  As  soon  as  the  balsam 
liquefies  (it  ought  on  no  account  to 
be  allowed  to  approach  ebullition) 
the  piece  of  stone  should  be  taken 
up,  reversed,  and  its  smooth  surface 
pressed  into  the  balsam  and  on  to 
the  surface  of  the  glass  slab  (fig.  27). 
The  slab  should  then  be  pushed  to  the  edge  of  the  hot  plate  so 

1  A  good  form  of  spirit  lamp  (fig.  28)  made  for  cooking  purposes, 
but  admirably  adapted  for  microscopical  mounting,  is  sold  by  Scott  & 
Son,  42  Bedford  Street,  Strand,  London,  and  costs  less  than  two  shil- 
lings. It  is  made  of  tinned  iron  ;  the  wick  is  stuffed  into  a  tinned  iron 
cylinder  an  inch  in  diameter,  and  a  cap  with  a  brass  collar  screws  over 
the  burner  when  it  is  not  in  use.  On  the  sides  of  the  lamp  are  three 
small  sockets  which  carry  bent  iron  wires,  thus  forming  a  strong  tripod 
upon  which  a  copper  or  iron  plate  can  be  placed  for  mounting. 


FIG.  28. 


Preparation  of  Sections.  67 

that  it  can  be  conveniently  removed  by  a  small  pair  ot 
tongs.  The  tongs  best  adapted  for  this  purpose  consist  of 
an  ordinary  wine-cork  cut  in  half,  the  separate  halves  being 
fixed  to  a  jointed  arm,  such  as  an  old  pair  of  compasses 
or  dividers  (fig.  29).  With  these  the  corner  of  the  slab 
should  be  firmly  held,  and  it  ought  then  to  FIG  ^ 
be  placed  upon  a  piece  of  wood  or  thick  paste- 
board laid  on  the  table  and  the  piece  of 
stone  firmly  squeezed  and  held  down  on  the 
glass  slab  until  the  balsam  begins  to  harden. 
For  this  purpose  the  cork  ends  of  the  tongs 
may  be  used,  as  the  stone  is  usually  too  hot 
to  be  fingered.  All  these  processes  must  be 
rapidly  performed.  The  slab  should  then  be 
taken  up  and  examined  from  its  under  side 
to  see  that  no  air-bubbles  have  been  included 
between  the  glass  and  the  stone.  Should  they  be  present 
in  any  quantity,  the  slab  must  be  again  placed  on  the  hot 
plate,  the  balsam  liquefied,  the  stone  detached,  and  both 
stone  and  glass  cleansed  from  balsam  by  means  of  tur- 
pentine or  benzol,  and  the  whole  process  of  cementing 
repeated,  as  otherwise  there  would  be  great  risk,  if  not  cer- 
tainty, of  the  stone  becoming  detached  from  the  slab  in  the 
later  stages  of  grinding,  and  it  is  far  better  to  undergo  a  little 
additional  work  in  this  way  than  to  run  the  chance  of  sacri- 
ficing all  the  labour  previously  devoted  to  the  preparation 
of  the  chip  and  that  subsequently  expended  on  the  second 
stage  of  grinding.  It  is  needful  to  impress  the  operator  with 
the  importance  of  using  hard  or  dry  balsam  in  cementing 
the  chip  to  the  slab,  otherwise  failure  is  almost  sure  to 
•ensue.  When  the  balsam  is  fairly  hardened  so  that  scraps 
of  it  surrounding  the  stone  can  be  scraped  off  and  rolled, 
almost  without  adhesion,  between  the  fingers,  the  slab  ought  to 
be  taken  up,  reversed,  and  the  adhering  stone  ground  again 
with  coarse  emery  and  water  on  the  iron  plate  or  on  the 
leaden  lap  of  the  machine.  When  the  stone  has  been  re- 

F  2 


68  The  Rudiments  of  Petrology. 

duced  sufficiently  to  transmit  light,  great  care  must  be  taken, 
and,  if  the  section  be  very  thin  (i.e.  if  the  stone  be  naturally 
rather  opaque  in  thin  plates),  the  pressure  upon  it  should  be 
diminished.  The  process  of  grinding  with  coarse  emery 
must  not  be  carried  too  far,  as,  when  the  section  is  ex- 
tremely thin,  it  may  often  be  entirely  removed  by  one  or  two 
turns  of  the  lap.  The  slab  and  its  adhering  section  should 
then  be  thoroughly  washed  and  freed  from  all  traces  of 
emery,  and  the  final  grinding  conducted  by  hand  on  the 
brass  plate  or  glass  slab  with  flour-emery  and  water.  In 
the  very  latest  stages  a  few  drops  of  paraffin  may  advan- 
tageously be  used  in  order  to  diminish  friction.  During  this 
final  grinding  the  section  should  be  frequently  examined 
under  the  microscope,  but  must  be  thoroughly  washed  and 
cleansed  prior  to  each  examination  and  a  drop  of  turpentine 
placed  on  its  surface  to  increase  its  transparency.  Now  is 
the  time  to  apply  tests  to  the  different  component  minerals, 
if  the  section  be  a  rock  or  an  impure  mineral,  and  doubt 
exist  as  to  the  nature  of  any  of  the  substances  present. 
Some  operators  employ  rouge  for  polishing  the  section  in  its 
very  last  stage  of  preparation,  using  a  piece  of  parchment  as 
the  surface  on  which  to  polish  it.  The  slab,  with  the  adher- 
ing section  uppermost,  must  once  again  be  placed  on  the 
hot  plate,  while  a  watch-glass  containing  turpentine  and 
placed  on  the  rim  of  a  pill-box  or  other  support  should  be 
near  at  hand.  When  the  balsam  is  thoroughly  liquid,  the  opera- 
tor should  take  the  slab  off  the  hot  plate  with  the  cork  tongs, 
and  by  means  of  a  blunt-ended  wire  (fig.  30),  held  in  the  right 
hand,  gradually  push  or  slide  the  section  off  the  slab  into 
the  turpentine  contained  in  the  watch-glass.  He  ought  then 
to  hold  the  watch-glass  by  means  of  a  wire  ring,  or  a  pair 
of  crucible-tongs  or  forceps,  over  the  lamp  or  gas  jet  suf- 
ficiently long  to  heat  the  turpentine  or  even  to  make  it  boil, 
but  the  watch-glass  must  be  kept  at  a  suitable  distance  from 
the  flame  to  prevent  ignition.  It  may  then  be  replaced 
on  its  support,  and  the  section  should  be  very  gently  turned 


Preparation  of  Sections,  69 

over  and  washed  in  the  turpentine  by  means  of  one,  or  two, 
small  camel's-hair  brushes.  A  glass  slide  (those  in  ordinary 
use  for  microscopic  preparations  are  perhaps  as  good  as  any 
for  size  ;  i  in.  by  3  in.)  must  then  be  placed  on  the  hot 
plate,  which,  however,  should  have  been  allowed  to  cool 
beforehand  ;  a  drop  FlG.  3<x 

of  ordinary  fluid  Ca- 
nada balsam  placed 

carefully  on  the  middle  of  the  slide  and  the  gas  or  lamp  lighted 
underneath.  The  section  should  then  be  taken  out  of  the 
turpentine  by  means  of  a  needle  mounted  in  a  handle  (fig.  31). 
This  may  be  done  FIG.  31. 

needle     underneath 

the  section  and  slowly  raising  it,  when  the  section  will  usually 
adhere  to  the  side  of  the  needle.  Superfluous  turpentine 
may  be  removed  from  the  section  by  touching  its  lower 
edge  with  the  finger,  FJG<  32. 

on  to  which  the  tur- 
pentine will  flow. 
During  this  short 
process  the  balsam  on  the  slide  ought  to  be  closely  watched 
to  see  that  it  does  not  get  overheated.  Should  it  boil  it  is 
useless  to  proceed  until  another  slide  with  fresh  balsam  has 
been  warmed  sufficiently,  without  boiling.  The  lower  edge 
of  the  section  should  then  be  placed  on  the  balsam,  the 
upper  edge  being  supported  by  the  needle,  and  it  should 
be  allowed  to  subside  gently  on  the  balsam,  so  that  no  air- 
bubbles  are  included.  The  slide  must  be  at  once  removed 
from  the  hot  plate,  or,  better  still,  it  should  be  removed  before 
laying  the  section  upon  it.  The  section  can  be  gently  moved 
about  by  means  of  a  needle,  to  induce  the  balsam  to  pass 
over  its  edges,  and  the  needle  may  then  be  used  horizontally 
to  drag  the  balsam  over  the  upper  surface  of  the  section. 
If  this  cannot  be  easily  accomplished,  an  additional  drop 
of  balsam  may  be  placed  on  the  top  of  the  section,  and  the 


7O  The  Rudiments  of  Petrology. 

whole  very  slightly  heated.  When  the  section  is  completely 
covered  with  balsam,  a  thin  and  clean  glass  cover  of  suitable 
size  should  be  taken  in  a  pair  of  forceps,  held  for  an  instant 
over  the  gas  or  lamp  flame,  and  be  let  down  gradually  on 
the  section.  It  should  be  gently  pressed  on  the  surface  to 
bring  it  close  to  the  section  and  to  drive  out  any  air-bubbles; 
a  slightly  rotatory  motion  will  be  found  useful  for  the  latter 
purpose. 

The  slide  may  then  have  most  of  the  superfluous 
balsam  scraped  off,  and  be  numbered  or  labelled  by  means 
of  a  writing-diamond,  and  set  aside  to  dry.  When  \vhat 
remains  of  the  superfluous  balsam  is  found  to  be  tolerably 
hard,  it  should  be  removed  as  much  as  possible  by  a  knife 
with  a  square  end,  fig.  32  (if  one  with  a  point  be  used  there  is 
danger  of  dislodging  the  cover),  and  the  remainder  may  be 
cleaned  off  with  a  rag  dipped  in  turpentine,  or,  better  still, 
in  benzol.  The  preparation  is  then  complete,  unless  the 
operator  likes  to  stick  paper  labels  on  the  ends  of  the  slide. 
The  writing  on  paper  labels  has  the  advantage  of  being  more 
easily  legible  than  diamond-writing,  but  brief  names  or 
numbers  should  always  be  first  scratched  on  the  glass  with 
a  diamond,  as  the  scratches  help  the  paper  label  to  adhere 
more  firmly,  and,  what  is  still  more  important,  they  are/^r- 
manetit  records,  so  that  if  labels  become  detached  the  sections 
can  still  be  identified.  Some  mounters  etch  the  names  and 
numbers  on  the  ends  of  their  slides  by  hydrofluoric  acid,  but 
the  marks  left  by  a  writing-diamond  are  more  legible  and 
take  far  less  time'  to  do.  In  preparing  sections  of  very  soft 
and  friable  rocks  the  following  process,  communicated 
to  me  by  Mr.  John  Arthur  Phillips,  may  be  had  recourse 
to  : — 

The  chip,  which  may  to  some  extent  be  previously 
hardened  by  saturation  in  a  mixture  of  balsam  and  benzol 
until  thoroughly  impregnated  with  it,  and  afterwards  dried, 
should  be  gently  ground  or  filed  down  until  a  smooth,  even 
surface  is  procured  ;  this  surface  must  then  be  attached 


Preparation  of  Sections.  7 1 

to  a  piece  of  a  glass  slide  cut  about  an  inch  square,  and  this 
again  fixed  in  a  similar  manner  by  old  balsam  to  a  thicker 
piece  of  glass  if  needful,  so  that  it  can  be  conveniently  held 
whilst  the  grinding  is  carried  on.  When  it  is  reduced  to 
such  a  degree  of  tenuity  that  it  will  bear  no  more  grinding, 
even  with  the  finest  materials,  such  as  jeweller's  rouge,  and 
when  the  removal  of  the  section  from  the  glass  to  which  it  is 
attached  would  almost  inevitably  result  in  the  destruction  of 
the  preparation,  the  lower  piece  of  glass  should  be  warmed  and 
separated  from  the  upper  piece  which  bears  the  section,  and 
this,  with  its  attached  section,  should  be  again  cemented  by 
the  under  side  of  the  glass  to  an  ordinary  glass  slip,  covered 
in  the  usual  way,  and,  if  the  edges  of  the  section,  or  its 
glass,  be  disfigured  by  grinding,  a  ring  or  square  margin  of 
Brunswick-black,  or  asphalt,  may  be  painted  over  the 
unsightly  part. 

Mr.  E.  T.  Newton,  the  Assistant  Naturalist  of  the  Geolo- 
gical Survey,  who  has  successfully  examined  the  microscopic 
structure  of  many  varieties  of  coal,  has  favoured  me  with  the 
following  notes  upon  the  methods  employed  by  him  in 
making  his  preparations  : — 

*  One  important  point  to  be  noticed  at  the  outset  is,  that 
nothing  like  eiitery  powder  can  be  used  for  the  grinding,  as 
the  grains  embed  themselves  in  the  softer  substance  of  the 
coal,  and,  when  the  section  is  finished,  will  be  seen  as  minute 
bright  spots,  thus  giving  to  the  section  a  deceptive  appear- 
ance. For  the  rough  grinding  an  ordinary  grindstone  may 
be  used,  and  for  the  finer  work  and  finishing,  a  strip  of 
*'  pumice  stone  "  (or  corundum  stick),  and  a  German  hone  (or 
water  of  Ayr  stone).  The  form  of  these  which  has  been 
found  most  convenient  is  a  strip  about  i£  inches  wide  and 
about  6  inches  long ;  the  thickness  is  immaterial  :  one  of 
the  broader  surfaces  of  these  must  be  perfectly  flat.  Having 
selected  a  piece  of  coal  with  as  few  cracks  as  possible,  cut  off 
a  piece  with  a  saw  about  f  of  an  inch  square  and  perhaps 
£  of  an  inch  thick.  One  of  the  larger  surfaces  is  then  rubbed 


72  TJie  Rudiments  of  Petrology. 

flat  on  the  pumice  stone,  keeping  it  well  wetted  with  water, 
and  then  polished  upon  the  hone,  also  moistened  with  water. 
Sometimes  it  is  found  to  be  advantageous  to  soak  the  piece 
of  coal  in  a  very  thin  solution  of  Canada  balsam  in  chloro- 
form or  benzol,  as  directed  for  softer  rocks  (p.  70),  or  in  a 
solution  of  shellac  in  spirits  of  wine  ;  in  either  case  allowing 
the  specimen  to  dry  thoroughly  in  a  warm  place.  The 
polished  surface  is  next  cemented  to  an  ordinary  microscopic 
glass  slip  (3  inches  by  i  inch)  with  the  best  marine  glue ;  and 
this  process  requires  care,  for  it  is  not  easy  to  exclude  all 
the  air-bubbles,  and,  if  they  are  not  excluded,  the  section  is 
very  apt  in  the  last  stages  to  break  av/ay  wherever  they 
occur.  The  piece  of  coal  is  next  reduced  to  about  y1-^  of  an 
inch  by  means  of  a  grindstone;  some  of  the  softer  kinds  may 
be  cut  down  with  a  penknife.  Care  should  be  taken  not  to 
scratch  the  glass  in  the  process  of  grinding,  for  most  sections 
of  coal,  when  once  ground  thin,  are  too  fragile  to  allow  of 
their  being  removed  from  the  glass,  but  have  to  be  covered 
and  finished  off  upon  the  same  slide.  The  pumice  stone  or 
corundum  stick  is  next  brought  into  use.  The  section  being 
turned  downwards,  hold  the  glass  slide  between  the  middle 
finger  and  thumb,  whilst  the  forefinger  is  placed  upon  the 
centre  of  the  slide.  In  this  manner  the  section  may  be 
rubbed  round  and  round  over  every  part  of  the  pumice, 
using  plenty  of  water,  until  it  is  sufficiently  reduced  in 
thickness ;  experience  alone  showing  how  far  this  process 
may  be  carried.  The  section  is  finally  rubbed  in  a  similar 
manner  upon  the  hone  (or  water  of  Ayr  stone).  It  is  some- 
times found  necessary  to  use  the  hone  even  while  the  sec- 
tion is  absolutely  opaque,  for  many  coals  are  so  brittle 
that  they  crumble  to  pieces  upon  the  pumice  long 
before  they  show  any  indications  of  transparency.  When 
sufficiently  transparent  the  section  may  be  trimmed  with 
a  penknife  and  the  superfluous  marine  glue  cleaned  off.  The 
section  is  now  to  be  moistened  with  turpentine,  a  drop  of 
ordinary  Canada  balsam  (not  too  hard)  placed  upon  it,  and 


Preparation  of  Sections.  73 

covered  in  the  usual  way.  Whatever  heat  is  necessary 
should  be  carefully  applied  to  the  cover  glass  by  reversing 
the  slide  for  a  moment  or  so  over  a  spirit  lamp,  otherwise 
the  marine  glue  may  be  loosened  and  the  section  spoiled. 
Balsam  dissolved  in  benzol  must  not  be  used  for  mounting, 
as  the  benzol  softens  the  marine  glue,  and  a  good  section 
may  in  this  way  be  destroyed.' 

When  no   section-cutting   or  grinding  apparatus  is   at 
hand  the  petrologist  may  sometimes  gain  a  rough  insight 
into  the   mineral  composition  of  a  rock  by  coarsely  pul- 
verising a  small  fragment,  and  examining  the  powder  under 
the  microscope.      In   such   crude    examinations   levigation 
may  occasionally  be  advantageous,  so  that  the        FIG 
minerals  of  different  specific  gravity  which  com- 
pose the  powder  may  be  examined  separately. 

In  the  case  of  very  soft  rocks  such  as  tuffs, 
clays,  &c.,  useful  information  may  sometimes  be 
acquired  by  washing  to  pieces  fragments  of  the 
rock ;  in  this  way  a  fine  mud  and  often  numerous 
minute  crystals  and  organisms  may  be  procured. 
The  best  apparatus  for  effecting  this  gradual 
washing  is  a  conical  glass  about  9  or  10  inches 
high,  across  the  mouth  of  which  a  cross-bar  of 
metal  or  wood  is  fixed.  A  little  hole  drilled  in 
the  centre  of  the  bar  receives  the  tube  of  a  small 
thistle -headed  glass  funnel.  Roughly  broken 
fragments  of  the  rock  should  be  placed  in  the  bottom  of  the 
conical  glass,  and  the  apparatus  set  beneath  a  tap,  from 
which  a  stream  of  water  is  continually  allowed  to  run  into 
the  mouth  of  the  funnel,  the  overflow  trickling  down  the 
sides  of  the  glass,  which  should  consequently  be  placed 
in  a  sink.  In  this  manner  a  constant  current  is  kept  up, 
and  the  fragments  at  the  bottom  of  the  glass  are  continually 
turned  over,  agitated,  rubbed  against  one  another,  and  gra- 
dually disintegrated.  This  action  should  be  kept  up,  often 
for  many  days,  until  a  considerable  amount  of  disintegrated 


74  The  Rudiments  of  Petrology. 

matter  has  accumulated.  Samples  should  then  he  taken 
out  by  means  of  a  pipette  and  examined  under  the  micro- 
scope. 

When  the  observer  wishes  to  mount  either  such  materials 
or  fine  scaly,  powdery  or  minutely  crystallised  minerals,  the 
best  method  is  to  spread  a  little  of  the  substance  on  a  glass 
slide,  moisten  the  powder  with  a  drop  of  turpentine,  and  then 
add  a  drop  of  Canada  balsam,  and  cover  in  the  usual 
manner.  If  the  attempt  be  made  to  mount  such  substances 
directly  in  balsam,  without  the  intervention  of  turpentine  or 
some  kindred  medium,  air-bubbles  are  almost  certain  to  be 
included  in  the  preparation. 


CHAPTER    IX. 

ON     THE    EXAMINATION     OF    THE    OPTICAL    CHARACTERS    OF 
THIN    SECTIONS    OF   MINERALS    UNDER   THE   MICROSCOPE. 

THE  phenomena  of  polarisation  under  various  conditions 
are  already  described  in  so  many  text  books  of  physics, 
some  of  which  are  specially  devoted  to  optics  and  others 
exclusively  to  the  study  of  polarisation,  that  for  general  in- 
formation on  the  subject  the  reader  is  referred  to  the  follow- 
ing works  : — 

'Manuel  de  MineValogie/  A.  Descloizeaux  (Introduction), 
p.  xxvi.  t.  i.  1862. 

'  Me'moire  sur  1'emploi  du  Microscope  polarisant.'  A.  Des- 
cloizeaux. 

'  Natural  Philosophy,'  A.  Privat  Deschanel.  Translation 
by  Everett,  pt.  IV.  p.  1032. 

'  Cours  de  Mineralogie.'  M.  De  la  Fosse,  ch.  xi.  t.  i.  Nou- 
velles  Suites  a  Buffon.  Roret.  Paris.  1858. 

'  Ganot's  Physics.'     Translation  by  Atkinson. 


Optical  Characters  of  Minerals.  7  5 

'  Lardner's  Natural  Philosophy — Optics,'  ch.  x.  London  : 
Walton  &  Maberley.  1858. 

'Elemente  d.  Petrographie/  A.  von  Lasaulx,  pp.  13-18. 
Bonn.  1875. 

'  The  Nature  of  Light,  with  a  General  Account  of  Physical 
Optics,'  by  Dr.  Eugene  Lommel.  International  Scientific 
Series.  1875. 

'  Lectures  on  Polarised  Light,'  delivered  before  the  Pharma- 
ceutical Society  of  Great  Britain,  by  J.  P(ereira).  Longman. 
1843. 

'  Mikroskopische  Physiographic  d.  petrographisch-wichtigen 
Mineralien,' by  H.  Rosenbusch.  1873.  Pp.  55-107. 

'  Polarisation  of  Light/  by  Wm.  Spottiswoode.  Nature 
Series.  1874. 

'  Notes  of  a  Course  of  Nine  Lectures  on  Light,'  by  John 
Tyndall.  1872. 

'  Six  Lectures  on  Light/  delivered  in  America,  John  Tyndall. 
2nd  edit.  1875. 

{ A  Familiar  Introduction  to  the  Study  of  Polarised  Light/ 
by  C.  Woodward.  Van  Voorst.  1861. 

'  Mikroskopische  Beschaffenheit  d.  Mineralien  u.  Gesteine/ 
by  F.  Zirkel,  p.  16.  Leipzig.  1873, 

'  Physikalische  Krystallographie.'     Groth.     Leipzig.    1876. 

For  the  purpose  of  investigating  the  optical  properties  of 
minerals,  various  instruments,  such  as  the  tourmaline  pincette, 
the  dichroscope,  the  stauroscope,  Norremberg's  polariscope, 
Descloizeaux's  polarising  microscope,  Rosenbusch's  stauro- 
microscope,  &c.,  have  from  time  to  time  been  devised.  The 
apparatus  most  commonly  employed  with  microscopes  con- 
sists of  two  Nicol's  prisms,  one  fitted  beneath  the  stage  of  the 
microscope  and  the  other  above  the  eye-piece  of  the  instru- 
ment or  above  the  objective,  the  lower  one  acting  as  the  pola- 
riser,  the  upper  one  as  the  analyser.  The  analyser  is  sometimes 
so  fitted  as  to  be  incapable  of  revolution,  but  the  polariser  is 
always  encased  in  a  tube  in  which  it  can  freely  revolve,  the 
needful  movement  being  imparted  directly  by  hand,  and, 
although  rack-work  is  fitted  to  the  sub-stages  of  some  mi- 


76  The  Rudiments  of  Petrology. 

•'  croscopes,  the  simpler  method  of  turning  the  polariser  is  by 
far  the  best.  It  will  also  be  found  most  advantageous  to 
have  the  analyser  fitted  in  a  cap  which  slides  over  the  eye- 
piece of  the  microscope,  for  it  is  desirable  that  there  should 
be  as  little  difficulty  as  possible  in  removing  and  replacing 
either  of  the  Nicols. 

When  the  principal  optical  sections  of  the  two  Nicols,  i.e. 
when  the  shorter  diagonals  of  the  two  prisms  coincide 
in  direction,  the  field  of  the  microscope  appears  clear  and 
well -illuminated.  When,  however,  they  are  set  at  right 
angles  to  one  another,  there  should  be  a  total  extinction  of 
light,  the  field  appearing  perfectly  dark.  In  intermediate 
positions  the  field  becomes  more  or  less  obscure,  the  obscu- 
rity increasing  as  the  principal  sections  of  the  Nicols 
approximate  to  an  angle  of  90°.  It  is  advantageous  to  have 
the  means  of  setting  the  two  Nicols  accurately  at  right- 
angles. 

Supposing  the  Nicols  to  be  crossed  and  the  field  to  be 
quite  dark,  a  slip  of  glass  or  a  thin  slice  of  rock-salt,  if  placed 
on  the  stage  of  the  microscope,  will  produce  no  change,  the 
field  still  remaining  dark.  This  is  owing  to  the  glass  and  the 
rock-salt  being  singly-refracting  or  isotropic  substances,  the 
one  amorphous  and  the  other  crystallising  in  the  cubic  sys- 
tem. All  truly  amorphous  substances,  i.e.  those  in  which  no 
crystalline  structure  is  developed,  are  singly-refracting,  so 
long  as  they  are  not  subjected  to  conditions  of  abnormal 
strain  or  sudden  and  unequal  changes  of  temperature  re- 
sulting in  corresponding  molecular  disturbances,  but  since 
such  phenomena  are  only  observed  in  thick  plates  of  glass, 
&c.,  and  are  engendered  by  artificial  means,  they  do  not 
specially  concern  the  petrologist.  Again,  not  only  rock-salt 
but  all  other  minerals  which  crystallise  in  the  cubic  system 
are,  with  one  or  two  doubtful  exceptions,  singly  refractive. 
If  we  interpose  between  crossed  Nicols  a  thin  section  of  a 
porphyritic  pitch  stone,  the  field  of  the  microscope  will  no 


Optical  Characters  of  Minerals.  77 

longer  appear  totally  dark.  The  matrix,  or  the  pitchstone 
itself,  is  vitreous  and  behaves  like  glass  in  being  singly- 
refracting  and  so  affords  a  dark  ground,  but  the  porphyritic 
crystals,  which  are  frequently  felspars  or  micas,  appear 
brightly  illuminated,  and  often  polarise  in  brilliant  colours. 
Microliths  or  crystals  too  minute  to  be  identified  with  any 
particular  mineral  species,  are  also  of  common  occurrence 
in  pitchstones,  forming  strings  or  streams,  their  longest  axes 
lying  in  more  or  less  definite  directions.  These  microliths 
also  frequently  polarise.  Such  doubly-refracting  crystals 
and  microliths,  therefore,  appear  brightly  illuminated,  while 
the  glassy  matter  in  which  they  are  imbedded  remains  dark 
between  crossed  Nicols. 

All  minerals  which  polarise,  in  other  words  which  exhibit 
double  refraction  or  are  anisotropic,  are  neither  amorphous, 
nor  do  they  crystallise  in  the  cubic  system.  It  does  not, 
however,  follow,  because  the  section  of  a  crystal  exhibits 
single  refraction,  that  it  therefore  belongs  to  the  cubic  system  ; 
the  direction  in  which  the  section  is  cut  must  be  taken  into 
consideration,  because  crystals  belonging  to  the  tetragonal 
and  the  hexagonal  systems  are  singly  refractive  when  viewed 
in  the  direction  of  the  principal  crvstallographic  axis,  in 
other  words  when  their  sections  coincide  with  their  basal 
planes.  This,  however,  is  the  only  direction  in  which  single 
refraction  occurs  in  the  minerals  of  these  doubly-refracting 
systems ;  they  are  consequently  spoken  of  as  optically  uni- 
axial.  The  crystals  of  the  three  remaining  systems,  viz., 
the  rhombic,  the  monoclinic,  and  the  triclinic,  are  opti- 
cally biaxial,  i.e.  there  are  two  directions  within  them 
along  which  single  refraction  takes  place.  With  regard  to 
single  refraction,  then,  we  may  sum  up  the  foregoing 
statements  in  the  following  manner  :  that  when  single 
refraction  is  exhibited,  i.e.  when  the  object  in  the  field  of 
the  microscope  remains  dark  between  crossed  Nicols,  the 
mineral  is  either  : 


78  The  Rudiments  of  Petrology. 


T3    D 

5^ 


Jj-s-l 

•  ~  bJO.2 
w5   oJ   C 

b^  8 
^S 


Amorphous  .  Singly  refractive  in  all  directions. 
Cubic  .  „  „  „ 

„      the  direction  of 
the     principal     crystallographic 


axis. 


axial.  ^  Hexagonal  J 

1\  Singly  refractive  in  one  or  the  other 
Rhombic  of  the  two  optical  axes,  or  singly 

Monoclinic  I     refractive  when  an  axis  of  elas- 
Triclinic  ticity  2  coincides  with  the  shorter 

J      diagonal  of  the  polariser. 


1  The  sections  parallel  to  the  basal  planes  of  hexagonal  crystals  may 
be  distinguished  by  the  adjacent  faces  affording  angular  measurements 
of  120°. 

2  The  following  notes  extracted  from  De  la  Fosse's  Cours  de  Mine- 
ratogie  will  to  some  extent  suffice  to  indicate  the  difference  which  exists 
between  an  optical  axis  and  an  axis  of  elasticity: — 'When  a  ray  of  light 
falls  normally  upon  a  face  which  is  perpendicular  to  a  certain  direction 
in  a  doubly-refracting  crystal,  it  becomes  separated  into  two  rays  which 
are  polarised  in  opposite  directions  and  which  are  propagated  with 
different  velocities  and  generally  in  different  directions,  but  there  are 
certain  special  directions  in  which  this  angular  separation  of  the  two 
oppositely  polarised  rays  does  not  take  place,   so  that  the  two  rays 
appear  to  coalesce  and  to  behave  as  a  single  ray;  and,  if  the  face  from 
which  they  emerge  be  also  at  right  angles  to  this  line,  the  two  rays  do 
not  separate  on  their  emergence,  and  consequently  an  object  viewed 
in  this  direction  through  two  parallel  faces  of  a  crystal  affords  a  single 
image.      If,    however,    emergence   take    place    from   a  face   situated 
obliquely  to  this  direction,  separation  of  the  two  rays  occurs  and  a 
double  image  is  formed.     The  particular  directions  which  fulfil  these 
conditions  are  the  axes  of  elasticity.     In  some  doubly-refracting  crystals 
there  are  only  three  of  these  axes,  in  others  there  is  an  infinity  of  them. 
The  axes  of  elasticity  may  therefore  be  defined  as  lines  along  which 
there  is  no  bifurcation  or  angular  separation  of\the  two  refracted  rays, 
under  the  conditions  of  incidence  just  enunciated  ;  and  the  distinctive 
character  of  an  axis  of  elasticity  consists  in  a  single  image  being  visible 
through  two  faces  perpendicular  to  this  axis,  and  in  a  double  image 
being  seen  through  faces,  one  of  which  (namely,  the  one  turned  towards 
the  object)  is  perpendicular  to  this  axis  of  optical  elasticity,  and  the 
other  oblique  to  it.     In  axes  of  elasticity  there  is  always  a  difference  in 
the  rate  of  propagation  of  the  polarised  rays,  which  is  very  great  com- 
pared with  that  which  takes  place  along  other  lines  among  rays  differ- 
ently polarised.     In  the  optical  axes,  on  the  contrary,  no  difference 
exists  in  the  rate  of  propagation,  which  is  absolutely  the  same  for  all 
rays  which  pass  along  them,  whatever  may  be  their  plane  of  polarisa- 
tion.    The  optic  axes  are  also  termed  axes  of  double  refraction,  since 
they  may  be  regarded  as  the  directions  in  which   double  refraction 


Optical  Characters  of  Minerals.  79 

In  doubly-refracting  minerals  it  is  therefore  evident  that 
chromatic  effects  are  produced  by  polarisation  only  when 
the  section  is  not  taken  at  right  angles  to  the  optical  axes. 
When  in  a  doubly-refracting  mineral  an  axis  of  elasticity 
(i.e.  a  direction  along  which,  under  certain  conditions,  a  ray 
of  light  is  polarised  in  two  opposite  directions  with  different 
velocities,  but  without  undergoing  bifurcation)  corresponds 
in  direction  with  the  shorter  diagonal  of  the  polariser,  no 
change  from  obscurity  is  visible  when  the  Nicols  are  crossed, 
but  on  revolving  the  object  in  a  horizontal  direction  it 
no  longer  remains  dark  but  transmits  light  and  polarises  in 
colours.  By  turning  the  object  in  a  horizontal  plane  or  in  a 
plane  at  right  angles  to  the  axis  of  vision,  it  is  therefore  pos- 
sible to  distinguish  singly-refracting  or  isotropic  from  doubly- 
refracting  or  anisotropic  minerals.  The  thickness  of  the 
section  under  examination  may,  however,  in  some  cases, 
especially  when  the  divergence  between  the  ordinary  and 
extraordinary  ray  is  slight,  become  too  limited  to  permit 
any  display  of  double  refraction. 

In  all  cases  it  is  necessary  to  rotate  the  section  between 
the  crossed  Nicols  before  it  can  be  definitely  settled  that 
the  mineral  under  examination  is  isotropic.  It  is  so  only 
when  it  remains  dark  in  all  phases  of  its  revolution. 

Supposing  a  section  placed  between  crossed  Nicols  to 
exhibit  colour,  and  that  a  complete  horizontal  rotation  of 
the  section  gives  rise  to  four  consecutive  changes  from  a 
condition  of  darkness  to  one  of  colour;  then,  when  the 
principal  directions  of  vibration  in  a  section  of  a  crystal  are 
parallel  and  at  right  angles  to  the  crystallographic  axes,  the 
section  in  question  may  be  tetragonal,  hexagonal,  rhombic, 
or  monoclinic. 

If  all  the  sections  of  the  same  mineral  do  not  behave 
alike,  the  mmeral  is  tetragonal,  hexagonal,  or  monoclinic. 

becomes  abolished — directions  in  which  a  bifurcation  of  the  rays  is  no 
longer  induced,  and  in  which  single  images  only  are  formed  instead  of 
the  double  images  which  are  produced  in  directions  along  which  a 
separation  of  the  rays  takes  place.' 


80  The  Rudiments  of  Petrology. 

If,  besides  the  sections  which  exhibit  colour  under  crossed 
Nicols,  there  are  others  of  the  same  mineral  which  appear 
dark,  and,  if  the  former  show  dichroism  and  pleochroism, 
the  mineral  is  then  tetragonal  or  hexagonal. 

If,  besides  the  sections  in  which  the  principal  directions 
of  vibration  are  parallel  and  at  right  angles  to  the  crystallo- 
graphic  axes  (sections  lying  in  the  zone  of  the  orthodiagonal), 
there  are  others  of  the  same  mineral  in  which  this  is  not  the 
case,  the  mineral  is  monoclinic,  and  it  may  also  exhibit 
pleochroism. 

When  all  the  sections  of  the  same  mineral  behave  simi- 
larly and  may  be  pleochroic,  the  mineral  is  rhombic. 

Again,  supposing  the  principal  directions  of  vibration 
are  neither  parallel  nor  at  right  angles  to  the  crystallographic 
axes,  and  that  pleochroism  may  occur,  we  then  have  either  a 
monoclinic  or  a  triclinic  mineral  to  determine. 

If  all  the  sections  do  not  behave  alike,  and  if  in  some 
cases  the  principal  directions  of  vibration  are  not  parallel 
and  at  right  angles  to  the  crystallographic  axes,  the  mineral 
is  then  monoclinic ;  but, 

If  all  the  sections  behave  alike,  it  is  triclinic. 

A  study  of  the  systems  of  rings  and  brushes  which 
constitute  the  interference  figures  of  crystals  examined  by 
means  of  a  convergent  pencil  of  polarised  light,  either  in  a 
Norremberg's  polariscope  or  in  a  polarising  microscope, 
such  as  that  of  Descloizeaux,  is  very  important  for  the 
precise  determination  of  the  crystallogra'phic  system  to 
which  minerals  belong ;  but  the  limits  of  this  book  preclude 
the  possibility  of  describing  either  the  apparatus  or  the 
phenomena,  and  the  student  is  therefore  referred  to  some  of 
the  works  on  polarisation  already  cited.1 

The  determination  of  the  crystallographic  system  to 
which  a  mineral  belongs,  and  the  exact  position  of  the 

1  Polariscopes  for  this  purpose  are  manufactured  by  Fuess,  Berlin ; 
Laurent  and  Lutz,  Paris  ;  Steeg,  Homburg;  Browning  and  Ladd, 
London,  and  some  other  opticians. 


Von  KobeWs  Stauroscope.  81 

planes  of  vibration  and  of  the  axes  of  elasticity,  are  best 
effected  by  means  of  the  Stauroscope.  The  simplest  form 
of  this  instrument  is  that  first  devised  by  Van  Kobell.  An 
improvement  upon  this  form  has  since  been  made  by 
Brezina ;  while,  for  the  stauroscopic  examination  of  thin 
sections  of  minerals  under  the  microscope,  it  is  only  needful 
to  have,  in  addition  to  the  usual  polarising  apparatus,  a 
rather  thick  plate  of  calcspar,  cut  at  right  angles  to  the 
principal  axis,  inserted  between  the  eye-piece  and  the  eye- 
piece analyser,  and  to  have  crossed  cobwebs,  for  centering 
the  object,  fixed  within  the  focus  of  the  eye-piece.  The 
microscope  used  for  this  purpose  should,  however,  possess 
an  accurately  graduated  and  concentrically-rotating  stage, 

FIG.  34.. 


so  that  the  angle  through  which  the  object  is  turned  can  be 
read  off  with  precision. l  It  will,  however,  in  the  first  place  be 
best  to  give  a  description  of  the  original  Stauroscope  of  Von 
Kobell.  This  consists  of  an  upright  A  (fig.  34),  fixed  to  a 
board,  and  carrying  a  metal  tube  B,  which  moves  in  a  vertical 
plane  by  a  joint  attached  to  the  top  of  the  upright.  The  tube 
carries  an  eye-piece  c,  consisting  of  a  Nicol's  prism  N,  which 
serves  as  the  analyser,  and  is  capable  of  rotation  about 
the  axis  of  the  instrument.  A  fixed  index,  D,  is  attached 
to  this  tube.  Another  tube,  E,  slides  within  the  tube  B 

1  Should  the  object  fail  to  fill  the  field  of  the  microscope,  a  perfo- 
rated metal  plate  may  be  superposed  in  order  to  cut  off  the  surrounding 
portion  of  the  field. 


82  The  Rudiments  of  Petrology. 

and  carries  a  semicircle  F,  which  is  divided  into  degrees. 
The  tube  E,  with  its  graduated  semicircle,  revolves  easily 
within  the  outer  tube  B,  and  the  angle  through  which 
it  is  made  to  revolve  may  be  read  off  by  means  of  the  semi- 
circle F  and  the  index  D.  Within  the  tube  E  slides  another 
short  tube,  o,  partially  closed  at  its  superior  extremity  by  a 
diaphragm ;  o  and  E  being  so  fitted  that  they  turn  together. 
Over  the  hole  in  the  diaphragm  of  o  the  plate  or  slice  of 
the  mineral  under  examination  is  fixed  with  wax.  A  plate 
of  calcspar,  cut  at  right  angles  to  the  principal  axis  of  the 
rhombohedron,  is  placed  in  the  upper  end  of  the  tube  B, 
just  belcrw  the  analyser.  A  parcel  of  blackened  plates  of 
glass,  P,  let  into  the  foot-board,  constitute  the  polariser. 
The  light  reflected  from  and  polarised  by  P  would,  if  nothing 
intervened  between  it  and  the  Nicol  N,  present  a  perfectly 
dark  field  when  the  plane  of  vibration  of  the  Nicol  was  set 
at  right  angles  to  the  plane  in  which  the  light  was  polarised 
from  P;  but  the  interposition  of  the  calcspar  plate  gives 
rise  to  an  interference  figure,  composed  of  a  concentric  series 
of  coloured  rings  intersected  by  a  dark  cross. 

In  using  the  instrument  it  must  be  so  arranged  that  the 
plane  of  vibration  of  the  analyser  N  is  at  right  angles  to  the 
plane  of  vibration  of  the  light  polarised  by  P,  and  this  must 
be  effected  by  turning  the  tube  E  until  the  zero  point  of  F 
lies  under  the  index  D.  The  object-carrier  o  is  then  taken 
out,  and  the  plate  of  the  mineral  to  be  examined,  the  faces 
of  which  should  be  smooth  and  parallel,  is  stuck  over  the 
hole  in  the  diaphragm  with  wax,  but  so  arranged  that  one 
of  its  edges  is  placed  parallel  to  an  engraved  line.  Two 
of  these  lines  are  engraved  on  the  upper  surface  of  the 
diaphragm  plate,  one  running  from  o°  to  180°,  and  the 
other  from  90°  to  270°.  The  cylinder  o  is  then  replaced 
in  E. 

Should  the  planes  of  vibration,  and  also  the  axes  of 
elasticity  in  the  mineral  plate,  be  parallel  to  the  planes  of 
vibration  of  the  analyser  and  polariser,  the  interference 


Brezincts  Stauroscope.  83 

figure  produced  by  the  calcspar  plate  will  undergo  no  dis- 
tortion. When  this  is  not  the  case  the  tube  E,  and  with  it 
the  tube  o  and  the  object  H,  must  be  turned  until  the  dark 
cross  is  perfectly  restored,  the  amount  of  the  revolution 
being  read  off  on  the  semicircle  F.  The  object-carrier  is 
then  removed  and  the  object  reversed,  so  that  its  other 
smooth  face  is  turned  towards  the  eye-piece,  care,  however, 
being  taken  that  it  is  readjusted  with  the  same  edge  on  the 
same  engraved  line  of  the  object- carrier.  On  being  once 
more  replaced,  the  tube  E  (with  its  contained  carrier  and 
object)  is  again  turned  until  the  dark  cross  of  the  interference 
figure  is  re-established,  the  angle  of  rotation  being  again 
noted,  the  mean  of  the  two  readings  giving  the  inclination 
of  the  observed  plane  of  vibration  to  the  selected  and 
adjusted  edge  of  the  mineral  plate.  Although  good  results 
are  to  be  obtained  with  this  instrument,  it  is  difficult  to 
perceive  any  marked  change  in  the  dark  cross  when  the 
object  is  only  turned  through  one  or  two  degrees,  while  the 

removal  of  the  obiect- 

J  FIG.  35. 

carrier  and  the  reversal 

of  the  object  is  also  ob- 
jectionable. To  obviate 
these  imperfections  Bre- 
zina  constructed  a  some- 
what different  instru- 
ment, employing  a  com- 
bination of  two  calcspar 
plates  cut  nearly  at  right 
angles  to  the  principal 
axis,  which  affords  a  very  sensitive  interference  figure,  indi- 
cated in  fig.  35.  The  middle  band  of  this  interference  figure 
becomes  dislocated,  as  shown  in  fig.  35  B,  when  the  axis  of 
elasticity  in  the  object  deviates  very  slightly  from  the  prin- 
cipal optical  section  of  the  analyser,  and,  if  the  method  of 
reversal  of  the  plate  be  adopted,  the  reading  of  the  angles 
of  rotation,  corresponding  with  the  two  displacements  of  the 

G  2 


84  The  Rudiments  of  Petrology. 

middle  band,  the  one  to  the  right,  and  the  other  to  the 
left,  will  afford  accurate  results  to  within  a  few  minutes. 

The  different   crystallographic   systems  may  be  deter- 
mined stauroscopically  in  the  following  way. 

(1)  If  the  plate  under  examination  be  amorphous,  or  if 
it  belong  to  the  cubic  system,  the  cross  of  the  interference 
figure  remains  unchanged  in  all  positions  of  the  plate. 

(2)  If  the  plate  belong  to  the  tetragonal  or  to  the  hexa- 
gonal system,  and  the  adjusted  edge  of  the  plate  under 
examination  be  situated  parallel  or  at  right  angles  to  the 
principal  axis  of  the  analyser,  the  cross  remains  unchanged. 
In  the  case  of  crystals  of  the  hexagonal  system,  there  is  also 
no  change  of  the  interference  figure  during  the  rotation  of 
the  object- carrier. 

Should  the  selected  edge  of  the  plate  not  be  adjusted  in 
the  way  just  described,  the  object  must  be  rotated  until  a 
sharply-defined  dark  cross  is  again  visible.  When  this  is 
the  case  parallelism  is  established  between  the  plane  of 
vibration  of  the  polariser  and  the  principal  axis,  or  a  direc- 
tion at  right  angles  to  the  principal  axis  in  the  mineral 
under  examination,  since  that  is  the  position  of  axes  of 
elasticity  in  uniaxial  minerals. 

(3)  In  the  rhombic  system,  when  the  plate  is  arranged 
at  zero,  the  cross  remains  unchanged  in  its  position  when 
any  one  edge  of  the  plate  parallel  to  a  crystallographic  axis 
is  adjusted  on  the  engraved  lines  of  the  object-carrier.     If, 
however,  the  selected  edge  have  not  this  position,  the  plate 
must  be  turned  until  the  interference  figure  is  restored ;  the 
angle  of  rotation  being  that  which  exists  between  that  edge 
and  an  axis  of  elasticity. 

In  the  rhombic  system  the  three  axes  of  elasticity  coin- 
cide with  the  three  crystallographic  axes. 

(4)  In  the  monoclinic  system  the  orthodiagonal  alone 
corresponds  with  one  of  the  axes  of  elasticity,  and  only  in 
the  case  where  an  edge,  parallel  or  at  right  angles  to  the 
o:thodiagonal,  is  adjusted  on  the  engraved  guide-lines  of 


Stauroscopic  Examination.  8  5 

the  object-carrier,  does  the  interference  figure  remain  un- 
changed at  zero.  If  the  adjusted  edge  be  in  any  other 
direction,  whether  parallel  to  the  vertical  axis  or  to  the 
clinodiagonal,  the  object  must  be  turned  until  the  cross  is 
restored,  the  angle  of  rotation  representing  the  angle  between 
the  adjusted  edge  and  one  of  the  axes  of  elasticity.  It  also 
gives  the  inclination  of  the  crystallographic  axis  to  the  axis 
of  optical  elasticity. 

(5)  In  the  triclinic  system,  the  axes  of  elasticity  and 
the  crystallographic  axes  bear  no  relation  to  one  another ; 
consequently,  when  a  crystallographic  axis  coincides  with 
the  plane  of  vibration  in  the  polariser,  the  interference 
figure  appears  distorted,  and  is  only  again  rectified  when  an 
axis  of  elasticity  is  turned  into  a  direction  parallel  to  the 
plane  of  vibration  of  the  polariser. 

When  an  axis  of  elasticity  in  the  plate  of  the  mineral 
under  examination  lies  parallel  to  the  plane  of  vibration  of 
the  polariser,  another  axis  of  elasticity  is  at  the  same  time 
parallel  to  the  principal  optical  section  of  the  analyser, 
since  the  optical  sections  of  the  two  Nicols  (if  two  Nicols 
be  used)  are  at  right  angles.  Under  these  circumstances 
the  cross  in  the  interference  figure  remains  unchanged. 

When  plates  of  a  mineral  can  be  examined  in  three 
directions  at  right  angles  to  one  another,  it  is  then  easy  to 
determine  the  crystallographic  system  of  the  mineral  by 
means  of  the  stauroscope.  In  microscopic  sections  of  rocks, 
sections  of  crystals  of  the  same  mineral  may  often  be  met 
with  in  the  three  directions  needful  for  a  complete  Stauro- 
scopic examination. 


86 


The  Rudiments  of  Petrology. 


CHAPTER  X. 

THE    PRINCIPAL    ROCK-FORMING    MINERALS  :    THEIR    MEGA- 
SCOPIC  AND    MICROSCOPIC    CHARACTERS. 

THE  following  list  comprises  the  minerals   which  usually 
occur   as   components   of  rocks.       Many  others  might  be 
added,  but  the  limits  of  this  work  preclude  the  possibility  of 
describing  a  larger  number.1 
(i)  Species     of     the     Felspar      (16)  Epidote. 
Group.  (17)  Sphene. 

(18)  Species   of  the  Garnet 

Group. 
(i9)*ToPaz. 

(20)  Zircon. 

(21)  Andalusite  and  Kyanite. 

(22)  Apatite. 

(23)  Rutile. 

(24)  Cassiterite. 

(25)  Calcspar. 

(26)  Quartz,  &c. 

(27)  Magnetite. 

(28)  Titaniferous  iron. 

(29)  Hematite. 

(30)  Limonite. 

(3 1 )  Iron  and  CopperPyrites. 

(32)  Zeolites. 

(33)  Viridite,  Opacite,  &c. 


-rf3)~"Leucite. 

(4)  Scapolite  and  Meionite. 

•(5)  Sodalite,      Hauyne,      and 

Nosean. 
X(6)  Olivine. 

(7)  Hypersthene. 

(8)  Enstatite. 

(9)  Bronzite. 

(10)  Species   of   the    Pyroxene 

Group. 

(11)  Species  of  the  Amphibole 

Group. 
-»-(i2)-  Species  of  the  Mica  Group. 

(13)  Chlorite. 

(14)  Talc. 

(15)  Tourmaline. 


SPECIES  OF  THE  FELSPAR  GROUP. 

Felspars   are    essentially   silicates    containing   alumina, 

together  with   potash,  soda,  baryta,2  lime,  or   any  two   or 

three  of  these  bases,  which  often  to  a  certain  extent  replace 

one  another  in  the  different  species.     With  few  exceptions, 

1  Some  short    but  admirable   notes  on  the  determination   of  the 
optical  characters  of  minerals  occur  in  a  paper  in  the  Bulletins  de  la  So- 
ciete  beige  de  Microscopic,   tome  iv.    1877-78,    entitled   '  Note  sur  un 
Microscope  destine  aux  Recherches  mineralogiques '  by  A.  Renard,  S.J. 

2  Baryta  occurs  in  the  species  hyalophane,  which  is,    however,  a 
mineral  of  comparative  rarity. 


Felspar  Group.  87 

the  felspars  are  white  or  of  pale  colour.  Except  when  in  a 
decomposing  or  decomposed  condition,  they  have  a  hard- 
ness of  about  6,  i.e.  they  can  be  scratched,  but  not  easily, 
with  the  point  of  a  knife. 

Chemically  they  may  be  divided  into  three  groups  :  the 
alkali,  the  lime,  and  the  mixed  alkali-lime  or  alkali-baryta 
felspars. 

Crystallographically  they  are  represented  in  two  systems  : 
the  potash  and  potash-baryta  felspars  occurring  in  the  mono- 
clinic,  and  the  others  in  the  triclinic  or  anorthic  system. 

Before  the  blowpipe  the  potash  felspars  fuse  with  diffi- 
culty, while  the  soda  felspars  fuse  more  readily.  Both  are 
insoluble  in  acids,  except  hydrofluoric  acid.  The  principal 
lime  felspars,  labradorite  and  anorthite,  are  both  of  them 
soluble  in  acids.  Labradorite  fuses  readily  before  the  blow- 
pipe, while  anorthite  is  more  difficultly  fusible  :  the  former 
fusing  at  3,  the  latter  at  5,  of  Von  Kobell's  scale. 

The  two  principal  directions  of  cleavage  in  the  mono- 
clinic  felspars  are  at  right  angles  to  one  another ;  those  of 
the  triclinic  felspars  lie  at  angles  oth^^jppo.  90°. 

Both  the  monoclinic  and  triclinifilKpars  have  a  perfect 
cleavage  parallel  to  the  basal  plane.  The  other  perfect 
cleavage  is,  in  the  monoclinic  felspars,  parallel  to  the  clino- 
diagonal ;  in  the  triclinic  it  is  parallel  to  the  brachy-diagonal. 
In  both  systems  there  are  hemiprismatic  cleavages  which 
are  more  or  less  imperfect,  and  more  perfect  in  one  direction 
than  in  the  other. 

When  the  light  falls  somewhat  obliquely  on  the  basal 
cleavage  plane  of  a  triclinic  felspar  it  is  usually  seen  to  be 
traversed  by  numerous  parallel  striations,  the  interspaces 
between  the  striae  representing  twin  lamellae.  This  twinning 
is  frequently  so  many  times  repeated  in  the  felspars  of  this 
system  that  more  than  fifty  lamellae  have  been  noted,  under 
the  microscope,  in  a  single  crystal.1 

1   '  Mikroskopische    Untersuchungen   liber   Diabase.'      Zeitsch.  d. 
deutsch.  geol.  Geselsch.,  Bd.  xxvi.  Heft  i.  p.  i,  by  J.  F.  E.  Dathe. 


The  Rudiments  of  Petrology. 
The  accompanying  diagrams  (fig.  36)  represent  how  these 

FIG.  36. 


FIG.  37. 


strise  are  due  to  hemitropy,  while  fig.  37  shows  the  appearance 

of  the  twin  lamellae  unler  polarised  light. 

The  absence  of  this 
striation,  however,  must 
not  always  be  taken  as  an 
indisputable  proof  that  the 
felspar  is  not  triclinic : 


nevertheless  it  is  most  common  in,  and  characteristic  of,  the 
felspars  of  this  system. 

When  the  light  falls  obliquely  either  on  the  basal  plane, 
the  orthopinakoid,  or  the  hemidome  of  a  monoclinic  felspar, 
a  simple  twinning,  as  evinced  by  difference  of  lustre,  may 
often  be  noticed  in  the  crystals  ;  this  twinning  takes  place 
upon  the  type  known  as  the  'Carlsbad  type.'  Twinning 
upon  other,  but  far  less  frequently  occurring,  types  is,  how- 
ever, also  known,  and  will  be  described  in  the  following 
pages. 

In  the  opinion  of  Tschermak,  the  species  albite  and 
anorthite  are  isomorphous,  the  soda  in  albite  being  repre- 


Felspar  Group.  89 

sented  by  lime  in  anorthite  :  intermediate  variations  occurring 
between  the  two  species  in  the  percentage  of  silica ;  six 
molecules  of  silica  occurring  in  the  formula  of  albite,  and 
only  two  in  that  of  anorthite. 

The  following  tables,  showing  the  formulae,  the  percentage 
composition,  and  the  oxygen  ratios  of  the  different  species, 
will  assist  the  student  in  learning  the  relations  which  they 
bear  to  one  another. 

CHEMICAL  FORMULA  OF  THE  PRINCIPAL  FELSPARS. 
Orthoclase  K2O  .  3SiO2  +  A12O3 .  3SiO2 
or  ??O     6SiO, 


A1203  ~ 


Albite 


or   {y1*^  6SiO2 
ALOa 


Anorthite     CaO  .  SiO2  + A12O3.  SiO3 


According  to  Tschermak's  view  3  molecules  of  albite  +  i 
molecule  of  anorthite  constitute  oligoclase  : 

3Na2O  +  3Al2O3+  i8SiO2  =  3  of  albite 
CaO  .....   +   A12O3+   2SiO2   =  i  of  anorthite 


CaO  +  3Na2O  +  4A12O3  +  2oSiO2  =  oligoclase 

i  molecule  of  albite  and  i  molecule  of  anorthite  constitute 
andesine  : 

Na2O  +  A12O3  +  6SiO2  =  i  of  albite 

CaO  .  .  .  .    +   Al2O3  +  2SiO2  =  i  of  anorthite 
CaO  +  Na2O  +  2A12O3  +  8SiO2  =  andesine 

i  molecule  of  albite  +  3  molecules    of  anorthite  constitute 
labradorite  : 

Na2O  +   A12O3  +   6SiO2  -  i  of  albite 

3CaO  .  .  .  .    +  3A12O3  +   6SiO2  *=  3  of  anorthite 
3CaO  +  Na2O  +  4A12O3  +  i2SiO2  =  labradorite 


9°  The  Rudiments  of  Petrology. 

PER-CENTAGE  COMPOSITION  OF  THE  PRINCIPAL  FELSPARS. 

Orthoclase  SiO2  =  64-20  A12O3  =  18-40  K2O    =16-95 
Albite  SiO2  =  68-6    Al2O3=i9'6     Na2O  =  ir8 


Anorthite     SiO2  =  43'i  A12O3  =  36'9  CaO  =20 

Oligoclase    SiO2  =  62'i  A12O3  =  237  Na2O  =  i4'2 

Labradorite  SiO2  =  52-9  A12O3  =  3O'3  CaO   =12-3 

Andesine      SiO2  =  597  A12O3  =  25'6  CaO   =   7 

OXYGEN  RATIOS  OF  THE  PRINCIPAL  FELSPARS. 
Orthoclase  .  Albite  .  Oligoclase  .  Andesine  .  Labradorite  .  Anorthite  . 

1:3:12    1:3:12    1:3:10       1:3:8         1:3:6         1:3:4 
being  respectively  the  oxygen  ratios  for  RO  .  R2O3  and  SiO2. 
RO  =  CaO  Na2O  and  K2O  .  and  R2O3  =  A12O3. 

In  this  manner  Tschermak  limits  the  number  of  species 
by  regarding  labradorite,  oligoclase,  and  andesine  as  admix- 
tures in  different  proportions  of  the  two  species  albite  and 
anorthite. 

The  following  extract  from  Dana's  '  System  of  Mineralogy,' 
5th  ed.,  1871,  p.  336,  may  here  be  cited,  as  pointing  out  the 
intercrystallisation  which  probably  gives  rise  to  the  com- 
pound-specific character  of  some  felspars.  After  giving  a 
list  of  the  oxygen  ratios  for  the  different  species,  he  adds  : 
'The  species  appear  in  the  analyses  to  shade  into  one 
another  by  gradual  transitions  ;  but  whether  this  is  the 
actual  fact,  or  whether  the  seeming  transitions  (when  not 
from  bad  analyses)  are  due  to  mixtures  of  different  kinds 
through  contemporaneous  crystallisation,  is  not  positively 
ascertained.  The  latter  is  the  most  reasonable  view.  It 
has  been  shown  by  Breithaupt  and  others  that  orthoclase 
and  albite  (or  the  potash  and  soda  feldspars)  occur  together 
in  infinitesimal  interlaminations  of  the  two  species,  and  that 
the  soda-potash  variety,  perthite,  is  one  of  those  thus  con- 
stituted. This  structure  is  apparent  under  a  magnifying 
power,  and  also  when  specimens  are  examined  by  means  of 


Felspar  Group.  91 

polarised  light.  Moreover,  these  and  other  feldspars  very 
commonly  occur  side  by  side  or  intercrystallised  when  not 
interlaminated,  as  oligoclase  and  orthoclase  in  the  granite 
of  Orange  Summit,  N.  Hampshire,  and  Danbury,  Conn.  ; 
in  obsidian  in  Mexico  ;  in  trachytes  of  other  regions.  Such 
facts  show  that  the  idea  of  indefinite  shadings  between  the 
species  is  probably  a  false  one,  since  the  two  keep  themselves 
distinct,  and,  in  the  perthite  and  similar  cases,  even  to  mi- 
croscopic perfection.  They  also  make  manifest  that  con- 
temporaneous crystallisation  is  a  true  cause  in  many  cases.' 

The  following  is  a  short  description  of  the  characters  of 
the  different  species  of  felspar  commonly  occurring  as  con- 
stituents of  rocks.  They  are  here  divided  into  two  groups 
according  to  the  systems  in  which  they  crystallise ;  since  it 
is  at  present  a  matter  of  considerable  difficulty  to  discriminate 
between  the  different  species  of  the  triclinic  system,  espe- 
cially when  the  crystals  are  so  minute  as  to  be  incapable  of 
isolation  for  the  purposes  of  chemical  analysis,  and  since 
all  the  felspars  which  crystallise  in  the  triclinic  system  pre- 
sent approximately  the  same  microscopic  appearances  under 
polarised  light.  The  two  groups  into  which  they  are  classed 
for  the  present  purposes  of  the  petrologist,  especially  when 
regarded  microscopically,  are  the  orthoclastic  (opOog  and 
K\ad)  =  rectangular  cleavage),  or  that  in  which  the  chief 
cleavages  are  mutually  situated  at  right  angles,  and  the 
plagioclastic(7rAay«oe  and  ^Xdw  =  oblique  cleavage),  or  those 
in  which  the  cleavage  planes  intersect  at  angles  other  than 
90°.  These  two  groups,  whose  members  are  respectively 
spoken  of  as  orthoclase  and  plagioclase,  may  in  most  cases 
be  readily  distinguished  under  polarised  light  by  the  dif- 
ferences which  they  present  in  their  twinning  ;  crystals  of 
orthoclase  and  its  varieties  usually  showing,  when  twinned,  a 
median  divisional  plane,  on  either  side  of  which  the  halves 
of  the  crystals  depolarise  the  light  in  complementary  colours; 
while  in  the  case  of  plagioclase  the  crystals  exhibit  numerous 
bands  of  different  colours.  When  sections  of  plagioclase 


92  The  Rudiments  of  Petrology. 

are  ground  very  thin,  their  twin  lamellae  usually  present  only 
pale  blue  or  neutral  tints  under  polarised  light ;  when  thicker, 
strong  colours,  often  variegated,  mark  the  different  lamellae. 
The  student  is,  however,  here  warned  that  conditions  may 
occur,  or  that  sections  of  crystals  may  be  so  cut,  that  these 
phenomena,  although  they  may  exist,  are  not  rendered  ap- 
parent. Doubtful  cases  also  occur  in  which,  at  times,  it  is 
very  difficult  to  assign  a  felspar  crystal  with  absolute  cer- 
tainty either  to  the  one  system  or  the  other. 

V  ORTHOCLASE. 

^  Crystalline  system  monoclinic  or  oblique.  The  fol- 
lowing figures  (38  and  40)  represent  the  common  forms. 
Fig.  39  shows  a  crystal  twinned  on  the  Carlsbad  type.  Figs. 
42,  43,  and  44  represent  the  twinning  of  fig.  41  upon  the 
Baveno  type ;  the  lines  T  T  indicating  the  planes  of  compo- 
sition. The  formulae  on  the  different  faces  are  those  of  Nau- 
mann.  In  the  variety  sanidine  the  orthopinakoid  is  usually 
less  developed  than  in  common  orthoclase. 

Angle  of  oblique  rhombic  prism  118°  48'.  Cleavage 
parallel  to  the  base  and  clinodiagonal  very  perfect  and  at 
right  angles  ;  parallel  to  one  or  other  hemiprism  imperfect. 

In  polarised  light  under  the  microscope  the  crystals 
sometimes  exhibit  moderately  strong  colours.  Crystals 
twinned  on  the  Carlsbad  type  are  of  common  occurrence ; 
and,  when  the  plane  of  section  coincides  with  the  ortho- 
pinakoid, they  polarise  in  different  colours  on  either  side  of 
a  median  line  which  represents  the  plane  of  composition, 
the  difference  in  colour  being  due  to  the  difference  of 
direction  of  the  optical  axes  in  the  opposite  halves  of  the 
crystal  and  the  positions  of  the  planes  of  chief  vibration 
in  the  Nicol's  prisms  of  the  polariscope,  the  difference  of 
direction  of  these  axes  being  due  to  hemitropy  or  a  half 
revolution  of  one  of  the  halves  of  the  crystal.  When  the 
section  is  cut  more  or  less  obliquely  to  the  orthopinakoid 
the  divisional  line  approaches  nearer  to  one  side  or  the 


Orthoclase. 


93 


other  of  the  crystal.       When  the  sectional  plane  almost 
coincides  with  the  clinopinakoid    only  a  narrow  marginal 


rig-,38 


OP 


band  of  a  different  colour  represents  one  of  the  halves  of 
the   crystal,   and,   when  it   coincides    completely  with   the 


94  The  Rtidiments  of  Petrology. 

clinopinakoid  all  signs  of  twinning  are  suppressed,  and  the 
crystal  presents  a  uniform  sheet  of  colour.  Variations  in 
the  uniformity  of  this  colour  are  then  due  to  a  corresponding 
want  of  uniformity  in  the  thickness  of  the  section.  Usually 
the  plane  of  composition  is  represented  by  a  straight  line, 
but  occasionally  it  appears  interrupted  and,  as  it  were, 
FIG  faulted  to  one  side  (fig.  45),  although  no  corre- 
sponding break  is  visible  in  the  boundary  lines  of 
the  crystal.  This  implies  that  the  apparent  shifting 
of  the  plane  of  composition  is  not  due  to  movement 
along  a  line  of  fracture ;  for,  if  so,  the  boundaries 
of  the  crystal  would  also  have  participated  in  the 
movement ;  but  it  must  rather  be  attributed  to  irre- 
gular interpenetration  of  the  two  halves. 

Weiss,  by  an  examination  of  orthoclase  crystals 
in  a  Norremberg's  polariscope,  has  shown  that  in  those  twinned 
on  the  Baveno  type  the  planes  of  the  optical  axes  (as  indi- 
cated by  the  interference  figures)  stand  at  right  angles  to 
one  another  in  the  two  halves  of  the  crystals. 

Crystals  of  sanidine  mainlv  differ 

FIG.  46.  ...  *  .  . 

in  microscopic  appearance  from  those 
of  ordinary  orthoclase  in  that  the 
former  are  clear  and  pellucid,  while 
the  latter  are  less  so,  often  having  a 
hazy,  turbid,  or  nebulous  aspect. 

In  the  massive,  cleavable  varieties 
of  orthoclase,  polarised  light  reveals 
a  very  peculiar  structure  which  is 
perfectly  evident  under  low  magni- 
fying powers.  The  mineral  appears 
to  be  broken  up  into  a  chequered 

Ortnoclase-Arendal  X  50.  r  . 

(Polar.)  mass  by  septa  of  varying    thickness 

and  colour  (fig.  46),  and  these  under  higher  powers  present  a 
somewhat  pectinate  appearance.  The  septa  or  striae  run  at 
right  angles  to  one  another,  so  that  the  structure,  expressed 
in  its  simplest  terms,  is  a  rectangular  reticulation.  Stelzner 


Orthoclase. 


95 


FIG.  47. 


has  pointed  out  that  one  of  these  directions  of  striation  is 
parallel  to  the  ortho-,  the  other  to  the  clino-pinakoid.  If 
this  be  the  case,  they  do  not  bear  any  direct  relation  to  the 
cleavage  of  the  mineral,  as  might  at  first  sight  be  imagined, 
and  an  examination  of  the  broken  edges  of  thin  sections 
does  not  seem  to  lend  much  support  to  such  a  supposition. 
This  structure,  although  pre-eminently  characteristic  of  mas- 
sive orthoclase,  is  not  so  frequently  seen  in  the  small 
crystals  imbedded  in  rocks.1  It 
is,  however,  often  to  be  observed 
in  little  microscopic  patches  in 
some  of  the  triclinic  felspars, 
notably  in  oligoclase  (fig.  47), 
thereby  indicating  that  these 
minerals,  as  suggested  by  Sterry 
Hunt2  and  Tschermak,3  are  by 
no  means  homogeneous.  Actual 
interlamination  of  albite  and 
thoclase  can  be  seen  with  the 
naked  eye  in  the  variety  of  fel- 
spar known  as  perthite,  but  in« 
most  of  the  other  felspars  in 
which  these  admixtures  of  spe- 
cies occur  the  orthoclase  does 
not,  as  a  rule,  form  laminae,  but  merely  lies  in  disconnected, 
irregular,  or  lenticular  patches  which,  however,  are  often 
disposed  more  or  less  in  the  general  direction  of  twinning 
which  the  species,  serving  as  matrix,  presents.  In  minute 
sanidine  crystals  occurring  in  some  vitreous  and  trachytic 

1  Some  of  the  orthoclase  crystals  in  the  hornblendic  granite,  of 
which  Cleopatra's  needle  is  made,  show  this  structure  very  distinctly. 

2  Chemical  and  Geological  Essays,  2nd  edition.     Salem,  1878^.443. 
Sterry  Hunt's  conclusions,    which  are  almost  identical  with  those  of 
Tschermak,  were  first  published  in  the  American  Journal  of  Science,  in 
Sept.  1854,  or  ten  years  previous  to  the  appearance  of  Tschermak's 
paper. 

3  Sit^mgsberichte  d.   Kais.   Akad.  d.  Wissenschaft.     Wien.   Bd.  I. 
Abth.  i.  571  (1864). 


Oligoclase-Twedestrand,  Norway 
x  115.     (Polar.) 


96 


The  Rudiments  of  Petrology. 


FIG.  48. 


rocks  other  peculiar  structures  are  sometimes  exhibited. 
These  consist  in  markings  which  are  usually  very  faint,  and 
often  necessitate  the  use  of  tolerably  high  magnifying  powers 
in  order  to  make  them  out  clearly.  In  such  crystals  divi- 
.sional  markings  or  internal  boundaries  may  be  discerned 
(fig.  48).  These  consist  sometimes  of  two  curved  lines,  with 
their  convex  aspects  directed  inwards, 
and  often  approximating  or  being  in 
actual  contact,  the  lines  seerm'ng  to 
spring  from  the  lateral  edges  of  the 
crystal,  while  at  others  they  consist  of 
a  straight  median  line  or  rib  which 
traverses  the  crystal  for  some  distance, 
and  then  suddenly  bifurcates,  the  bifur- 
cations passing  in  straight  lines  to  the 
opposite  lateral  edges  or  corners  of 
the  crystal  section.  These  larger  divi- 
sional markings,  whether  curved  or 
rectilinear,  are  crossed  by  other  mark- 
ings or  striae,  which  are  usually  very 
numerous,  often  extremely  delicate, 
and  always  observe  definite  direc- 
tions ;  but,  as  the  angles  made  by  their 
intersections  vary  very  considerably  in  different  crystals,  it 
is  unsafe  at  present  to  hazard  any  conjectures  as  to  the 
relation  which  they  bear  to  general  recognised  crystalline 
structure.1 

Microcline — In  the  blue,  chatoyant  felspar  of  the  zircon 
syenite  of  Norway  the  angles  of  intersection  of  the  principal 
cleavages  are,  according  to  Breithaupt,  90°  22'  to  90°  23',  so 
that  in  this  respect  it  differs  slightly  from  the  cleavage  of 
orthoclase. 

Descloizeaux  has  found  this  mineral  to  be  optically  similar 

1  'Notes  on  some  Peculiarities  in  the  Microscopic  Structure  of 
Felspars.'  F.  R.  Quarterly  Journal  Geological  Society,  p.  479, 
1876. 


Sanidine  crystal  in  rhyolite. 
Berkum,  Rhine,  x  115. 
(Polar.) 


Albite.  97 

to  orthoclase  and  he  has  appropriated  Breithaupfs  name  to 
the  green  felspar  know  as  Amazon-stone,  which  in  certain 
varieties,  especially  those  from  Colorado  and  Arkansas,  ex- 
hibits optical  properties  incompatible  with  monoclinic  sym- 
metry, while  in  other  physical  respects,  and  chemically,  it  is 
not  distinguishable  from  orthoclase.  These  facts  are  of 
special  interest  as  more  completely  establishing  the  isomor- 
phism of  orthoclase  and  albite,  a  pure  potash  felspar  of 
triclinic  symmetry  having  been  previously  unknown. 

Descloizeaux  has  pointed  out1  that  thin  sections  of 
Amazon-stone,  when  magnified,  are  seen  to  inclose  bands  and 
patches  of  albite,  yet  although  such  albite  inclosures  are 
very  numerous  and  comparatively  large  in  some  examples, 
the  percentage  of  soda  which  they  yield  on  analysis  is  always 
slight. 

Perthite,  which  presents  a  well-marked  interlaminated 
structure  to  the  naked  eye,  consists  of  differently  coloured 
alternating  bands  of  orthoclase  and  albite. 


Crystalline  system  triclinic  or  anorthic.  The  cleavages, 
which  are  parallel  to  the  base  and  brachypinakoid,  intersect 
at  angles  of  86°  24'  and  63°  36'.  The  cleavage  faces  usually 
have  a  pearly  lustre. 

A  fine  lamellar  or  twinning  striation  is  often  visible  on 
the  basal  plane.  The  plane  of  composition  is  parallel  to 
the  brachypinakoid.  Untwinned  crystals  are  rare.  In  che- 
mical composition  it  is  essentially  a  soda-felspar.  It  is 
usually  white  or  greyish. 

Albite,  on  losing  its  alkaline  constituents,  passes  into 
kaoiin,  &c.,  just  as  orthoclase  does. 

Before  the  blowpipe  it  fuses  with  difficulty  to  a  whitish 
glass,  and  colours  the  flame  yellow.  It  is  not  acted  upon 
by  acids. 

Sections  of  crystals,  so  long  as  they  do  not  coincide  with, 

1  Annales  de  Chimie  et  de  Physique,  $me  Serte,  I.  ix.  433,  1876. 
H 


The  Rudiments  of  Petrology. 


or  approximate  very  closely  to,  the  plane  of  the  brachypina- 
koid,  exhibit  under  the  microscope  coloured  parallel  bands 
when  viewed  by  polarised  light.  Sections  of  albite  seldom 
contain  any  microliths  or  other  inclosures. 

Figures  49  and  50  illustrate  the  albite  type  of  twinning, 
while  in  fig.  5 1  the  direction  of  the  plane  of  composition  in 
the  twinning  of  pericline  is  shown.  The  lines  TT  indicate 


Tiff.  51, 


the  planes  along  which  the  twinning  takes  place.  Fig.  52 
shows  a  group  of  three  twinned  crystals  of  triclinic  felspar, 
such  as  are  often  seen  under  the  microscope.  Fig.  53  shows 
the  abrupt  termination  of  twin  lamellse  occasionally  to  be 
observed  in  plagioclastic  felspars. 

v  ANORTHITE. 

This  is  a  mineral  which  is  not  of  very  frequent  occur- 
rence. 


Oligoclase^  Andesine ,  and  Labradorite.  99 

The  best  crystals  are  found  in  the  Vesuvian  lavas. 

Its  crystalline  system  is  triclinic  or  anorthic. 

It  cleaves  parallel  to  the  basal  plane  and  to  the 
macropinakoid  ;  the  cleavages  intersecting  at  an  angle  of 
94°  12',  and  exhibiting  a  pearly  lustre.  Twin  striation 
is  not  as  a  rule  strongly  marked  on  the  basal  cleavages. 
The  plane  of  twinning  corresponds  with  that  of  albite. 

Before  the  blowpipe  it  fuses  with  difficulty  to  a  clear 
glass.  It  is  completely  soluble  in  concentrated  hydrochloric 
acid.  When  imbedded  in  rocks  the  crystals  usually  have  a 
greasy  lustre,  but  when  formed  in  druses  they  are  generally 
glassy  and  limpid. 

r  /  y 

OLIGOCLASE,  ANDESINE,  AND  LABRADORITE. 

All  of  these  species  crystallise  in  the  triclinic  system,  and 
are  traversed  by  two  sets  of  cleavage  planes,  the  one  parallel 
to  the  basal  plane,  the  other  parallel  to  the  brachypinakoid, 
which  intersect  at  angles  of  86°  10'  and  93°  50'.  In  colour 
,  they  vary  from  white  to  different  shades  of  grey,  while 
labradorite  is  sometimes  almost  black,  as  that  from 
Hamilton  Sound,  Labrador.  Labradorite  also  shows  a 
fine  play  of  variegated  colours  on  planes  parallel  to  the 
brachypinakoid,  and  occasionally  oligoclase  likewise  exhibits 
a  play  of  colour. 

The  lustre  of  oligoclase  and  andesine  is  vitreous,  ap- 
proaching to  greasy  on  the  basal  cleavage,  while  that  of 
labradorite  is  vitreous  and  somewhat  pearly. 

Oligoclase  is  not  acted  upon  by  acids,  except  hydro- 
fluoric acid.  It  fuses  more  readily  than  orthoclase  before 
the  blowpipe,  and  colours  the  flame  yellow. 

Andesine  is  only  imperfectly  acted  upon  by  acids,  except 
hydrofluoric  acid.  The  edges  of  thin  splinters  may  be  fused 
before  the  blowpipe. 

Labradorite  in  a  fresh,  unaltered  condition  is  only  im- 
perfectly soluble  in  hydrochloric  acid,  but  weathered  samples 

H  2 


loo  The  Rudiments  of  Petrology. 

are  completely  decomposed  by  it,  with  separation  of  gela- 
tinous silica. 

The  three  species  of  felspar  now  under  consideration 
present  such  closely  analogous,  or,  according  to  observations 
hitherto  made,  identical,  characters  under  the  microscope, 
that  by  this  means  alone  it  is  at  present  impossible  to  dis- 
criminate between  them,  and  consequently  they  are  all 
described  under  the  general  term  '  plagioclase '  by  micro- 
scopists. 

By  one  or  two  ready  methods  of  analysis,  which  have 
been  devised  for  the  purpose  of  identifying  very  minute 
quantities,  and  of  superseding  the  more  tedious  processes  of 
ordinary  chemical  analysis,  the  different  species  may,  how- 
ever, be  identified.  Amongst  these  methods  may  be  cited 
that  of  Szabo,  based  upon  the  relative  duration  of  colour 
imparted  to  the  flame  of  a  Bunsen's  gas  jet  by  two  assays  of 
known  weight,  and  which  may  be  denned  as  a  system  of 
colour-comparison ;  and  a  new  method  of  chemico-micro- 
scopic  investigation  recently  introduced  by  Prof.  Boficky, 
of  Prag,  which  consists  in  treating  the  substance  with  hydro- 
fluo-silicic  acid  and  examining  under  the  microscope  the 
different  crystalline  forms  of  the  fluo-silicates  produced  by 
the  use  of  this  reagent ;!  a  system  resting  upon  the  deter- 
mination of  the  forms  and  chemical  composition  of  artificially 
produced  crystals.  In  cases  where  these  artificially  formed 
crystals  belong  to  the  same  crystallographic  system,  but  may 
differ  in  chemical  composition,  reagents  other  than  hydro- 
fluo-silicic  are  employed,  and  the  crystals  formed  by  these 
subsequent  reactions  give  confirmatory  evidence  of  the 
chemical  nature  of  the  previously  undetermined  fluo-silicates, 
which  resulted  from  the  first  reaction  with  hydro-fluo-silicic 
acid. 

Prof.  Boficky  informs  the  author  that  he  has  recently 
been  conducting  experiments  upon  this  system  on  minute 

1  '  Elemente  einer  neuen  chemisch-mikroskopischen  Mineral-  und 
Gesteinsanalyse.'  Boficky  (Archiv  d.  Natunv.  Landesdurchforschung 
von  Bohmen.  ///,  Band.  Chem.-petrologische  Abtheilung),  1877. 


Borickfs  Method  $f  A  ncfiyys.  I  o  i 

sections  of  minerals  occurring  in  thin  microscopic  slices  of 
finely  crystalline  rocks,  and  that  from  fragments  or  imbedded 
crystals  not  more  than  o-2  millemeters  to  07  millemeters 
square,  he  has  procured  good  results,  which  '  are  often 
remarkably  beautiful,  so  that  even  minerals  in  very  fine- 
grained rocks  may  be  separately  examined.' l 

The  limits  of  this  work  preclude  the  possibility  of  anything 
more  than  a  brief  allusion  to  this  method  of  investigation. 

The  hydro-fluo-silicic  acid  used  for  this  purpose  is 
made  in  a  leaden  retort  from  fluoride  of  barium,  sulphuric 
acid,  and  pure  powdered  quartz ;  the  resulting  fluo-silicate 
is  then  transferred  to  a  platinum  dish  containing  water,  and 
after  moderate  dilution  is  decanted  into  a  gutta-percha 
bottle.  Minute  quantities  of  the  reagent  are  applied  to  the 
minerals  under  examination  by  means  of  a  gutta-percha  rod 
terminated  by  a  little  spoon-shaped  groove.  The  strength 
of  the  solution  used  by  Prof.  Boficky  is  about  3^  per  cent. 
It  is  important  that  neither  too  weak  nor  too  strong  a  solu- 
tion be  used,  since  in  the  former  case  many  minerals  do  not 
afford  any  satisfactory  reactions,  and  in  the  latter  so  many 
crystals  of  fluo-silicates  are  formed,  and  from  many  silicates 
so  much  silica  is  separated,  that  the  field  of  the  microscope 
becomes  filled  with  an  indistinct,  hazy  mass,  in  which  no 
definite  crystalline  forms  can  be  distinguished.  In  such  a  case 
further  dilution  with  one  or  two  drops  of  water  is  necessary. 

The  fragment  under  examination  may  be  about  the 
size  of  a  pin's  head  or  a  millet  seed.  A  drop  or  two  of 
Canada  balsam  should  be  placed  on  a  glass  slip  and  heated, 
but  not  to  ebullition,  and  the  slip  should  then  be  turned 
about  so  as  to  run  the  balsam  into  a  thin  even  sheet. 
Upon  this  surface,  when  cooled  and  hard,  is  placed  the 
small  fragment  of  the  mineral  to  be  examined,  and  the  slide 
is  again  sufficiently  heated  to  cause  adhesion  of  the  frag- 
ment A  drop  of  the  hydro-fluo-silicic  acid  solution  should 
then  be  applied,  and  the  slide  set  aside  in  a  horizontal  position 
1  Private  communication  from  Prof.  Boficky. 


IO2  The  Rudiments  of  Petrology. 

on  a  flat  plate  and  in  a  place  free  from  dust,  a  small  capsule 
containing  sulphuric  acid  being  placed  beside  it,  and  the 
whole  covered  with  a  small  glass  shade  or  an  inverted 
tumbler ;  a  dry  atmosphere  is  thus  insured,  but  in  spite 
of  this  it  often  takes  24  hours  before  the  drop  is  com- 
pletely evaporated.  When  evaporation  is  over,  the  prepara- 
tion is  ready  for  examination  under  the  microscope. 

Crystals  of  fluo-silicate  of  sodium  are  cubic ;  those  of 
magnesium,  iron,  and  manganese  are  hexagonal  or  rhombohe- 
dral ;  those  of  lithium,  strontium,  and  calcium  are  monoclinic. 

The  fluo-silicates  of  sodium,  magnesium,  and  calcium 
are  so  different  in  form  that  they  can  at  once  be  distinguished 
from  one  another.  Again,  the  fluo-silicates  of  lithium  differ 
sufficiently  from  those  of  strontium  and  calcium  to  render 
their  recognition  easy.  The  distinction  between  the  crystals 
of  fluo-silicates  of  calcium  and  strontium,  and  between 
those  of  magnesium,  iron,  and  manganese  is,  however, 
scarcely  possible,  and  other  reactions  must  be  had  recourse 
to  in  order  to  determine  their  respective  chemical  natures. 
Thus,  for  example,  if  the  fluo-silicates  of  calcium  and 
strontium  be  treated  with  somewhat  dilute  sulphuric  acid, 
the  crystals  of  the  former  substance  will,  after  a  few  seconds, 
become  surrounded  with  a  thick  fringe  of  monoclinic,  aci- 
cular  crystals  of  gypsum,  while  the  crystals  of  fluo-silicate  of 
strontium  pass  slowly  into  granular  masses,  interspersed 
with  short  needles  (of  celestine  ?),  a  process  which  usually 
takes  several  hours. 

In  polarised  light,  under  the  microscope,  crystals  of 
plagioclase  show,  as  already  stated,  a  series  of  parallel  bands 
or  twin  lamellae,  which  polarise  in  various  colours ;  and  this 
appearance  of  plagioclase  is  very  characteristic  so  long  as 
the  crystals  are  not  cut  parallel  to  the  brachypinakoid. 

Sections  of  labradorite  crystals  cut  parallel  to  the  macro- 
pinakoid  sometimes  show  another  system  of  striations  or 
interlamellar  growths;  these  form  outcrop  striae  on  the 
brachypinakoidj  while  the  ordinary  twin  lamellae  crop  out 


Labradorite.  103 

on,  and  striate,  the  basal  plane.'.  The  latter  represent  twin- 
ning upon  the  albite  type,  and  the  others  possibly  indicate 
the  pericline  type  of  twinning.  Sections  taken  parallel  to 
the  basal  plane  would  show  the  former  but  not  the  latter ; 
sections  parallel  to  the  brachypinakoid  would  show  the 
latter  but  not  the  former ;  while  in  sections  parallel  to  the 
macropinakoid  both  systems  of  lamellae  would  be  visible, 
and  their  true  angle  of  intersection,  which,  according  to 
Dr.  A.  von  Lasaulx,  is  86°  40',  could  be  correctly  measured. l 
These  lamellae  are  usually  sufficiently  broad,  and  those  per- 
taining to  the  pericline  type  of  twinning  are  generally  so 
irregularly  developed,  or  their  planes  of  demarcation  are 
distributed  at  such  wide  intervals,  that  there  is  little  fear  of 
mistaking  such  intersecting  twinning-planes  in  labradorite 
for  the  rectangular  cross-hatching  which  occurs  in  orthoclase. 
If  the  labradorite  crystals  were  very  small,  and  their  sections 
did  not  offer  the  means  of  measuring  precisely  the  angle  of 
intersection  of  the  two  sets  of  lamellae,  some  such  doubt 
might  be  entertained,  but  it  so  happens  that  this  twinning 
upon  the  pericline  type  is  as  a  rule  only  developed  in  plagio- 
clase  crystals,  which,  microscopically  speaking,  are  of  con- 
siderable dimensions  ;  while  on  the  other  hand  the  cross- 
hatching  in  orthoclase  is  developed  even  in  very  micro- 
scopically minute  crystals  and  interstitial  patches. 

In  sections  of  oligoclase  minute  imbedded  patches  of 
orthoclase  may  frequently  be  recognised  under  the  micro- 
scope, the  orthoclase  exhibiting  the  characteristic  cross- 
hatching  in  polarised  light.  These  patches  are  usually  very 
irregular  in  form,  but  are  often  distributed  in  more  or  less 
rudely  parallel  lines,  corresponding  with  the  direction  of  the 
twin  lamellae  in  the  plagioclastic  portion  of  the  mineral.2 

Microcline,  as  recently  shown  by  Delesse,  must  also  be 
regarded  as  an  admixture  or  interlamellation  of  albite  and 
orthoclase. 

1  Elemente  der  Petrographie.     A.  von  Lasaulx.      1875,  P-  44- 

2  Tschermak,   Siteungsberichte  d.  Kais.   Akad*  d.    Wiss.       Bd.    I. 
Abth.  i.  571.    Wien,  1864.    Also  Q.  J.  G.  S.,  vol.  xxxi.  p.  479.  F.  R. 


1 04  The  Rudiments  of  Petrology. 

The  similarity  or  identity,  which  has  already  been  pointed 
out,  in  the  microscopic  characters  of  the  different  species  of 
felspar,  comprised  under  the  general  head  of  plagioclase, 
renders  further  description  of  them  needless,  or  at  all  events 
unsatisfactory,  in  the  present  state  of  microscopical  know- 
ledge, and  the  determination  of  the  species  of  plagioclase 
must  therefore  rest  rather  on  their  chemical  deportment  than 
upon  their  microscopic  characters. 

NEPHELINE. 

This  mineral,  which  is  of  common  occurrence  in  many 
\  lavas,  and  notably  represents  the  felspathic  constituents  of 
3  many  basalts,  is  one  of  the  essential  components  of  phonolite. 
It  is  very  closely  allied  to  the  felspars  in  its  chemical  com- 
position A12O3  SiO2  +  RO    SiO2.      RO  representing  soda 
and  potash,  the  former  averaging  about  16  or  17  per  cent., 
and  the  latter  about  5  per  cent. 

Nepheline  has  a  conchoidal  or  uneven  fracture  and  a 
hardness  of  5*5  to  6. 

Before  the  blowpipe  it  fuses  to  a  colourless  glass,  and 
when  powdered  and  treated  with  acids  it  gelatinises. 

Nepheline  crystallises  in  the  hexagonal  system  in  rather 
stout  prisms  which  are  terminated  by  basal  planes,  and  are 
frequently  modified  by  planes  of  the  hexagonal  pyramid. 
There  are  two  directions  of  cleavage,  one  basal,  the  other 
prismatic,  but  neither  of  these  cleavages  is  perfect.  The 
FIGS  54  &  crystals  are  generally  colourless,  or  with  a  slight 
tinge  of  green,  yellow,  or  brown,  and  have  a 
vitreous  or  greasy  lustre. 

Sections  of  nepheline  crystals  when  cut  par- 
allel to  the  vertical  axis  afford  rectangular  forms 
whose  boundaries  are  constituted  by  the  planes 
co  P  and  oP,  while  those  cut  at  right  angles  to 
the  vertical  axis,  i.e.  parallel  to  the  basal  plane, 
appear  as  well-defined  hexagons.  Sections  cut  obliquely 


Sections  of  Hexagonal  Prism. 

PLATE  I. 


105 


io6  The  Rudiments  of  Petrology. 

to  the  vertical  axis  give  various  forms,  often  difficult  to 
refer  to  any  precise  direction.  Thus,  if  a  section  be  taken  in 
the  plane  SS  (figs.  54  and  55),  the  resulting  figure  will  be 
bounded  by  eight  sides ;  but  a  section  taken  parallel  to  the 
vertical  axis  of  a  modified  crystal  consisting  of  the  com- 
bination P,  co  P  and  oP  (fig.  56),  would  also  give  a  similar 
eight- sided  figure.  Sometimes  square  sections 
of  nepheline  are  met  with.  These  probably 
represent,  as  suggested  by  Zirkel,  prisms  of 
equal  breadth  and  length. 

Some  idea  of  the  various  forms  which  may 
result  from  sections  taken  in   different  direc- 
tions through  a  hexagonal  prism  may  be  acquired  from  an 
examination   of  the    figures  in   the   accompa- 
nying Plate  I.,  in  which  basal  planes  (o  P)  are 
indicated  by  thick  lines,  and  prismatic  faces 
(oo  P)  by  thin  lines.     These  figures,  however, 
only  roughly  indicate  the  forms,  without  any 
pretension  to  geometrical  accuracy.1 

If  thin  sections  of  nepheline  be  cut  par- 
allel to  the  principal  crystallographic  axis  and  examined  in 
polarised  light  under  the  microscope,  they  are  seen  to  polarise 
in  rather  feeble  colours,  and  which  are  never  so  strong  as  the 
colours  displayed  by  quartz,  since  the  double  refraction  of 
nepheline  is  comparatively  weak ;  greyish,  pale  blue,  yellowish, 
and  brownish  tints  are  the  most  common.  Should  a  rectan- 
gular section  of  nepheline  remain  dark  between  crossed 
Nicols,  it  is  only  necessary  to  turn  it  on  its  own  axis  in  order 
to  procure  a  display  of  colour.  If  transverse  sections  of 
nepheline  crystals  cut  parallel  to  the  basal  planes  be  exa- 
mined under  similar  circumstances  they  will  merely  appear 
translucent  when  the  principal  sections  of  the  Nicols  coin- 
cide, and,  upon  revolution  either  of  the  polariser  or  analyser, 

1  It  may  be  useful  for  the  student  to  make  outlines  of  all  the  possible 
forms  of  sections  which  may  be  derived ;  first,  from  the  simple  forms 
occurring  in  the  different  crystallographic  systems  and,  afterwards,  from 
observed  combinations. 


Nepheline.  107 

they  will  gradually  darken  until  the  principal  sections  of  the 
Nicols  are  crossed  when,  if  the  sections  be  truly  parallel  to 
the  basal  planes,  perfect  obscurity  sets  in.     Fluid  lacunae \\ 
and  other  inclosures  are  of  frequent  occurrence  in  nepheline.  j 

Among  the  latter  pale  greenish  or  yellowish  microliths  I 
and  granules  of  augite  are  common.     Microliths  of  horn-  ' 
blende  are  comparatively  rare,  although  they  do  occur  in  \ 
the  nephelines  of  some  phonolites. 

These  microliths  are  usually  seen  to  lie  in  more  or  less 
distinct  zones,  corresponding  with  the  boundaries  of  the 
sections  in  which  they  occur,  but  in  the  case  of  some  of 
the  larger,  transverse,  hexagonal  sections  they  have  been  ob- 
served to  assume  three  directions  corresponding  with  the 
lateral  axes  of  the  crystals. l  A  fine  dusty  matter  of  a  brown 
or  bluish  colour  is  often  met  with  in  nepheline,  especially 
when  it  occurs  in  hornblendic  rocks.  Under  a  high  magni- 
fying power  the  dust  is  seen  to  consist  of  very  diminutive 
microliths,  air  bubbles,  and  glass  lacunae.  This  dust  is  often 
very  densely  accumulated  in  zones  which  run  parallel  to  the 
hexagonal  boundaries  of  transverse  sections,  while,  at  times 
it  is  also  segregated  in  the  centre  of  the  crystals.  Nepheline 
crystals  with  dark  centres,  which  often  occupy  almost  the 
entire  area  of  a  transverse  section,  may  be  seen  in  the  pho- 
nolite  of  the  Wolf  Rock,  which  lies  off  the  coast  of  Corn- 
wall.3 The  fluid  inclosures  in  nepheline  have  in  some  cases 
been  regarded  as  liquid  carbonic  acid,  and  at  times  they 
contain  movable  bubbles  and  also  minute  crystals  ;  those 
observed  by  Sorby  in  nepheline  from  Vesuvius  were  cubic, 
and  were  considered  to  be  either  chloride  of  sodium  or  of 
potassium,3  The  alteration  visible  in  weathered  crystals  of] 
nepheline  consists  of  a  fibrous  pale  yellowish  substance,] 
which  is  first  developed  in  the  exterior  portions  of  the  crystals,) 

1  Zirkel,  Mik.  Beschaff.  d.  Min.  und  Gest.,  p.  143. 

2  This  rock  was  first  described  by  S.   Allport  in  the    Geological 
Magazine,  Decade  I.  vol.  viii.  p.  247. 

3  Sorby,   « On  the  Microscopical  Structure  of  Crystals,'  Q.  y.  G.  S., 
vol.  xiv.  p.  480. 


io8  The  Rudiments  of  Petrology. 

\  and  gradually  extends  inwards./  The  ultimate  result  of  such 
alteration  is  a  zeolitic  substance  which  is  possibly  natrolite.1 

El&olite  is  a  greenish,  brownish,  sometimes  reddish 
variety  of  nepheline.  It  has  an  oily  lustre  and  fuses  more 
readily  before  the  blowpipe  than  nepheline.  It  seldom  or 
never  occurs  crystallised  as  a  rock  constituent  but  in  a 
massive  or  simply  crystalline  condition.  As  observed  by 
Rosenbusch,  it  bears  the  same  relation  to  nepheline  that 
orthoclase  does  to  sanidine.  Under  the  microscope  it  is 
seen  to  contain  numerous  greenish  inclosures,  some  pulve- 
rulent and  referred  to  diaspore  by  Scheerer,  while  Rosen- 
busch, Zirkel,  and  others,  have  observed  plates  and  microliths 
of  hornblende  in  Scandinavian  and  American  elseolites.  To 
their  presence  the  colour  and  peculiar  lustre  of  the  mineral 
have  been  attributed,  but  Rosenbusch  regards  the  latter 
character  as  due  to  special  molecular  structure.  Elseolite 
is  a  constituent  of  the  rocks  zircon- syenite,  foyaite,  mias- 
cite,  and  ditroite. 

Cancrinite  is  probably  an  altered  condition  of  nepheline. 
Plates  of  hornblende,  similar  to  those  in  elseolite,  have 
been  observed  in  this  mineral.  It  sometimes  contains  a 
colourless  alteration-product  which,  under  the  microscope, 
exhibits  aggregate  polarisation.  It  is  a  component  of  the 
rock  named  ditroite  in  which  it  occurs  associated  with  soda- 
lite,  elseolite,  orthoclase,  &c. 

•   LEUCITE  (AMPHIGENE). 

Leucite,  until  within  the  last  few  years,  lias  been  be- 
lieved to  crystallise  in  the  cubic  system,  its  form  closely  re- 
sembling that  of  analcime.  In  1872,  however,  G.  vom  Rath 
\announced2  that  it  crystallised  not  in  the  cubic  but  in  the 
(tetragonal  system.  '  His  conclusions  were  based  upon  the 
occurrence  of  striae  which  denote  planes  of  twinning  and 
which,  when  they  extend  to  the  edge  of  a  face,  pass  across 

1  A.  von  Lasaulx,  Elements  der  Petrographie,  p.  71. 

2  Report  of  British  Association  (Brighton),  1872,  p.  79. 


Leucite. 


109 


the  edge  and  are  continued  on  the  adjacent  face  without  any 
deviation  from  the  direction  of  striation  on  the  former  face. 
If  the  leucite  crystal,  therefore,  be  regarded  as  a  regular 
icosi-tetrahedron,  as  formerly  supposed,  this  plane  of  stria- 
tion would  cut  off  the  symmetric  solid  angles  of  the  crystal, 
and  the  plane  of  twinning  must  consequently  be  regarded  as 
parallel  to  a  face  of  the  rhombic  dodecahedron.  Twinning 
parallel  to  this  face  is,  however,  incompatible  with  cubic 
crystallisation,  and  leucite  crystals  cannot,  therefore,  belong 
to  the  cubic  system.  Following  out  this  deduction  by  care- 
ful measurements,  Vom  Rath  arrived  at  the  FIG  g 
conclusion  that  the  form  assumed  by  leucitej 
is  a  combination  of  a  di-tetragonal  pyramid; 
4?2  with  a  tetragonal  pyramid  P,  as  indicated^ 
in  the  accompanying  figure  58.  By  this  means 
the  supposed  anomalous  optical  characters  of 
the  mineral  are  accounted  for,  and  the  lamellae 
seen  under  polarised  light  in  microscopic  sections  may  now 
be  safely  regarded  as  twin  lamellae.  They  lie  in  planes 
parallel  to  the  face  2Poo  and  appear  sometimes  as  broad,7 
sometimes  as  excessively  nar- : 
row,  bands  of  bluish  grey  or  ; 
neutral  tints,  differing  in  in-  j 
tensity  (fig.  5  9),  and  no  strong 
chromatic  effects  are  ever; 
produced  by  their  polarisa-j 
tion.  In  such  polysynthetic 
crystals  the  twinning  planes 
lie  in  four  directions.  Al- 
though this  twin  structure  is I 
almost  invariably  present  and 
well  defined  in  moderately 
large  crystals,  the  minute  or 
purely  microscopic  crystals  of  leucite  frequently  exhibit  no 
trace  of  it/ as  in  the  little  leucite  crystals  of  the  sperone 
or  leucitophyr  which  occurs  near  Rome.  Were  it  not  for 


FIG.  59. 


110 


The  Rudiments  of  Petrology. 


'the  inclusions,  and  the  symmetrical  disposition  which  these 
inclusions  usually  assume,  such  diminutive  crystals  of  leucite 
might  easily  be  overlooked,  or  mistaken  for  some  other  mine- 
ral ;  and,  if  this  fact  be  borne  in  mind,  it  is  quite  possible  that 
leucite  may  yet  be  detected  in  rocks  in  which  its  existence 
has  never  been  suspected.  According  to  the  present  state  of 
our  knowledge,  it  is  a  mineral  of  very  restricted  occurrence, 
being  mainly  confined  to  a  few  comparatively  limited  areas^x 
"such  as  the  neighbourhood  of  Vesuvius,  Rome,  the  Eifel, 
Saxony,  Bohemia,  the  island  of  Bawian,  north  of  Java,  and 
In  Wyoming,  U.S. /Sections  of  very  minute  crystals  of  leucite 
frequently  show  ill-defined  boundaries,  so  that  their  appear- 
ance is  more  that  of  a  rounded  granule  than  of  a  properly 
developed  crystal.      Leucite  generally  affords  well-defined 
eight-sided  sections  which  are  mostly  clear,  transparent,  and 
:  always  colourless.     They  nearly  always  contain  inclosures  of 
either  colourless  or  brownish  glass,  dark,  opaque  granules 
of  magnetite  microliths  of  felspar  and  augite,  and  sometimes 
granules  of  the  latter  mineral.    These  inclosures 
are  generally  disposed  in  a  symmetrical  manner 
in  zones  which  lie  parallel  to   the  boundaries 
of  the  crystal,  but  the  zones  often  fail  to  coincide 
precisely  with  the  boundary,  and  they  then  ap- 
pear as  circles  (fig.  60). '   At  other  times  they  are  differently 
arranged  as  shown  in  fig.  61.     Frequently  micro- 
liths, granules,  and  glass  pores  are  all  present  in 
one  zone,  but  they  are  often  arranged  in  a  syste- 
matic manner,  the  microliths  and  glass-inclosures 
alternating  with  one  another.     Sometimes  these 
inclosures  are  congregated  in  the  centre  of  the  crystal  either 
in  symmetrical  groupings  (fig.  62),  or  else  huddled 
together  without    any   definite   arrangement;   at 
other  times   they  are    scattered    promiscuously 
throughout  the  crystal.    Occasionally,  but  rarely, 
minute  rods  of  glass  occur  in  leucite  crystals ; 
sometimes  they  are  short  and  thick  with  rounded  ends,  and 


FIG.  60. 


FIG.  61. 


FIG.  62. 


Sodalite,  Hauyne.  1 1 1 

they  are  often  incrusted  with  granular  matter^figs.  63  a  and 
63  c\  Occasionally  extremely  delicate  rods  are  visible  which 
traverse  the  leucite  for  long  distances  in  perfectly  straight 
lines,  then  end  abruptly  in  nodes  of  granular  matter  (fig.  63  £), 

FIG.  63. 

C 

I  rC& 


and  often  appear  to  be  continued  in  another  direction  in  a 
series  of  fine,  straight  rods,  thus  forming  a  zig-zag  arrange- 
ment, nodes  of  granular  material  marking  the  points  where 
they  divaricate.1 

SCAPOLITE  (PARANTHINE,  WERNERITE). 

This  mineral  occasionally  occurs  in  granite  and  in  some 
metamorphosed  limestones.  Its  chemical  composition  is 
represented  by  the  formula  2A12O3,  3SiO2  +  3(RO,  SiO2),  in 
which  RO  mostly  signifies  K2O,  but  generally  Na2O  is  also 
present,  and  at  times  traces  of  CaO  are  met  with.  Scapolite 
crystallises  in  the  tetragonal  system,  the  common  forms 
being  the  tetragonal  and  di-tetragonal  prisms  terminated  by 
pyramids.  It  also  occurs  massive  and  granular.  Before  the 
blowpipe  it  intumesces  and  fuses  to  a  white  glass.  It  is 
only  imperfectly  soluble  in  hydrochloric  acid.  Its  colour  is 
white,  bluish,  greenish,  and  occasionally  reddish.  It  is 
sometimes  transparent,  but  more  frequently  opaque.  It 
cleaves  parallel  to  the  faces  of  the  prisms.  The  cleavage  is 
more  perfect  parallel  to  oo  P  oc  than  in  the  planes  parallel  to 
oo  P.  The  hardness  is  about  5-5. 

Scapolite  has  strong  double  refraction.  Transverse  sec- 
tions of  scapolite  crystals  exhibit  well-marked  fissures  parallel 
to  the  cleavage  oo  P  oo.  Scapolite  rock  is  a  granular  mas- 
sive aggregate  of  the  mineral  together  with  orthoclase. 
Under  the  microscope  scapolite  is  usually  seen  to  contain 
1  Trans.  Royal  Mic.  Soc.,  vol.  xv.  p.  180. 


H2  The  R  udimen  ts  of  Petrology. 

greenish  alteration-products  which  sometimes  have  a  fibrous 
structure  at  right  angles  to  the  cleavage  planes. 

The  species  meionite  and  marialite  are  closely  Delated  to 
scapolite.  . 

SODALITE,  HAUYNE,  AND  NOSEAN 

are  all  silicates  of  alumina  and  soda,  and  each  in  addition 
contains  another  compound  which  serves  to  characterise  the 
several  species;  thus, 

Sodalite  contains  2NaCl  \ 

Hauyne       „       2CaSO4  I  +  3(Na2O,  SiO2  +  A12O3  SiO2). 

Nosean        „        Na2SO4j 

All  these  three  minerals  crystallise  in  the  cubic  system, 
the  usual  form  being  the  rhombic  dodecahedron.  Since 
they  all  crystallise  in  the  cubic  system  they  are  all  singly 
refractive.  They  all  cleave  parallel  to  faces  of  the  rhombic 
dodecahedron. 

Before  the  blowpipe  sodalite  fuses  with  intumescence  to 
;  a  colourless  glass ;  when  fused  with  microcosmic  salt  and 
oxide  of  copper  it  gives  the  blue  colouration  to  the  flame, 
characteristic  of  the  presence  of  chlorine.  Hauyne  and 
nosean  when  fused  with  carbonate  of  soda  yield  the  usual 
reaction  for  sulphuric  acid. 

All  three  of  these  minerals  are  decomposed  by  acids 
with  separation  of  gelatinous  silica.  In  the  case  of  hauyne 
sulphuretted  hydrogen  is  evolved  when  the  mineral  is  treated 
with  sulphuric  acid,  but  nosean  under  similar  circumstances 
evolves  none. 

Under  the  microscope,  sections  of  nosean  are  four-sided 
or  six-sided,  but  the  outlines  are  often  irregular.  The  crystals, 
when  fresh  and  unaltered,  appear  clear  and  colourless  in  the 
interior  of  the  sections,  but  are  bordered  by  a  somewhat 
broad  and  dark  external  band  which  coincides  with  the 
boundaries  of  the  section  ;  occasionally  crystals  exhibit  a 
concentric  series  of  these  bands.  The  junction  of  this  dark 
border  with  the  clear  internal  portion  of  the  crystal  is  rather 


Nosean  and  Hauyne.  1 1 3 

sharply  defined  (fig.  64).  Sometimes  a  clear  narrow  outer 
zone  surrounds  the  dark  border,  and  a  dark  spot  is  often  to 
be  seen  in  the  very  centre  of  the  crystal. 
The  clearer  portions  show,  under  a  tolerably 
strong  magnifying  power,  variable  quantities  of 
fine  dark  dust,  and  dark  but  very  fine  striae 
which  cross  one  another  at  angles  of  60°, 
90°,  and  120°.  Under  still  higher  powers  the 
striae  are  seen  to  consist  of  rows  of  minute  glass 
inclosures  and  opaque  granules.  The  dark  borders  and 
zones  of  the  nosean  crystals  are  also  seen  to  be  composed 
of  these  inclosures  very  closely  aggregated,  while,  at  times, 
whole  crystals  appear  nearly  opaque  owing  to  the  great 
quantity  of  these  inclosures  and  the  multiplicity  of  the 
striae  formed  by  them.  It  may  here  be  suggested  that 
the  clear  spaces  in  nosean  crystals  are  due  to  the  segre- 
gation of  these  dusty  inclosures  towards  boundaries  which 
represent  successive  zones  of  accretion,  and  that  this 
segregation  implies  the  consequent  abstraction  of  the  dust 
from  those  portions  which  ultimately  appear  clear ;  a  pro- 
ceeding analogous  to  that  which  seems  to  have  taken 
place  in  some  tachylytes  as  observed  both  by  the  late 
Herman  Vogelsang  1  and  by  the  author.2  Acicular  micro- 
liths,  opaque  granules  and  glass,  and  fluid  lacunae  are  the 
principal  inclosures  met  with  in  nosean.  When  the  mineral 
undergoes  alteration  it  becomes  yellowish  and  more  or  less 
obscure,  a  fibrous  and  at  times  radiate  structure  supervenes, 
and  the  whole  crystal  passes  into  zeolitic  matter  which 
polarises  in  variegated  colours.  Nosean  occurs  in  some 
phonolites,  basalts,  leucitophyrs,  nephelinites,  &c. 

The  sections  afforded  by  crystals  of  hauyne  are,  like 

1  Die  Krystalliten,  p.  112.     Bonn,  1875. 

'  '  On  the  Microscopic  structure  of  Tachylyte  from  Slievenalargy, 
Co.  Down,'  F.  Rutley,  with  an  analysis  by  Dr.  S.  Haughton.  Proc. 
Royal  Irish.  GeoL  Soc.,  1877. 


1 14  The  Rudiments  of  Petrology. 

those  of  nosean,  six-sided  or  four-sided.  The  microscopic 
structure  of  the  two  species  shows  in  many  cases  no  appre- 
ciable difference,  but  it  is  not  so  constant  in  character  in 
hauyne  as  it  is  in  nosean.  Sometimes  crystals  of  hauyne 
contain  scarcely  any  inclosures.1  Hauyne  usually  has  a 
stronger  blue  colour  than  nosean,  especially  in  the  darker 
zones,  but  a  blue  colouration  may  be  imparted  to  nosean  by 
heating  the  mineral  as  shown  by  Dressel : 2  the  phenomenon 
being  due  to  the  change  which  the  contained  sulphide  of 
sodium  undergoes,  and  in  the  rocks  of  the  Laacher  See  in  the 
Eifel,  which  contain  nosean,  hauyne  is  found  as  a  substitute 
in  those  ejected  blocks  and  lapilli  which  bear  traces  of 
fusion.  These  facts,  as  pointed  out  by  Dr.  A.  von  Lasaulx, 
of  Breslau,  indicate  the  probability  that  hauyne  and  nosean 
are  mere  varieties  of  the  same  mineral  species. 

The  red  colour  of  some  hauyne  is  due  to  the  presence  of 
scales  of  peroxide  of  iron  which  are  regarded  by  Zirkel  as  of 
secondary  origin.3  Crystals  of  hauyne,  like  those  of  nosean, 
contain  minute  glass  inclosures  and  dusty  matter,  which 
constitute  fine,  microscopic  striae  which  follow  the  directions 
of  the  crystallographic  axes.  Both  blue,  bluish- grey,  and 
colourless  matter  occur  in  some  crystals  of  hauyne,  the  diffe- 
rent colours  occasionally  marking  definite  zones,  but  more 
usually  forming  irregular  flecks  and  patches  of  colour  which 
bear  no  relation  to  crystallographic  form;  the  boundary 
between  the  darker  blue  and  the  lighter  colourless  matter 
is,  however,  ill  defined,  and  never  shows  a  sharp  line  of 
demarcation. 

The  crystals  of  hauyne  which  occur  in  the  basalts  of  the 
Laacher  See,  usually  have  a  broad  dark  border  which  shades 
off  towards  the  interior  of  the  crystal  into  a  bluish-grey  and 

1  Elements  der  Petrographie,  A.  von  Lasaulx,  p.  73.     Bonn,  1875. 

2  Neues  Jahrbttch  fiir  Mineralogie,  p.  565,  1870. 

«  Mikroskopische  Besch.  der  Min.  u.  Gesteine,  F.  Zirkel,  p.  163. 
Leipzig,  1873. 


, 

Sodalite.  /  >  1 1 5 

/  /' 

more  or  less  translucent  matter  (fig.  65),  xbut  income  very  j 

small  crystals  no  clear  area  is  visible  in  the  Centre,  '  /,', 
even  in  excessively  thin  sections,  the  whole  GT/S 
tal  being  nearly   or   quite  opaque.     The  striae 
the  quadrangular  sections  cross   one    another   at 
right   angles.      Crystals    of    hauyne  from     other 
localities  possess  a  clear  border  with  sharply-defined  internal ' 
and  external  boundaries,  the    inner    portion  of     F          • 
the  crystal   being  crossed  by  striae  which  some- 
times intersect  at  right  angles  (fig.   68),    and  at 
others  follow  three   directions,   each   set  of  striae 
forming  a    series  of    parallel    lines  which    run 
either  at  right  angles  to  the  opposite  faces  of  the  crystal 
(fig.  67),  or  pass  in  directions  which  would  corre-    F 
spond  with  lines  drawn  between  opposite  angles  of 
the  six-sided  section  (fig.  66).     In  both  cases  the 
striae  do  not  intersect,  but  are  divided  either  by 
dark  or  clear  lines  which  radiate  from  the  centre 
of  the  crystal ;  in  the  former  case  joining  opposite  angles, 
and  in  the   latter  passing  from  the  centre  to  the 
middle  of  the  faces.     The  striae  are  found  under 
high  amplification  to  consist  of  opaque  granules, 
gas  pores,  and  minute  glass  inclosures. 

Sodalite  occurs  in  rocks  either  in  an  uncrystal- 
lised  condition,  or  in  crystals  which  yield  six- sided  or  else  qua- 
drangular sections ;  the  latter  are  very  frequently  distorted,  so 
that  the  alternate  or  opposite  sides  of  the  section  are  unequal. 
They  usually  present  a  yellowish,  grey,  or  blue  colour.  The 
rhombic-dodecahedral  cleavage  is  represented,  under  the 
microscope,  by  undulating  cracks.  The  crystals  sometimes 
appear  to  be  remarkably  pure,  at  others  they  are  crowded 
with  various  inclosures ;  the  most  common  being  steam 
pores.  A  singly-refractive  substance  containing  fixed  bubbles 
also  occurs  in  some  sodalite  and  this  singly-refracting  matter, 
which  frequently  has  a  dark  border,  forms  well  defined 

I  2 


1 1 6  The  Rudiments  of  Petrology. 

rhombic  dodecahedra  which  contain  them;  they  do  not, 
however,  contain  any  bubbles.  Fluid  lacunae  which  change 
on  the  application  of  heat  are  found  sparingly  in  the  sodalite 
of  Somma  (Vesuvius)^  while  complete  included  crystals  often 
contain  inclosures  of  nepheline,  augite,  meionite,  and  biotite. 

OLIVINE   *• 

I  is  a  common  constituent  of  many  eruptive  rocks,  in  which 
/  it  occurs  sometimes  in  the  form  of  crystals  whose  angles 
'  1  frequently  appear  more  or  less  rounded,  and  sometimes  as 
1  rounded  granules  which  in  some  cases  form  rounded  aggre- 
gates, occasionally  showing  traces  of  crystal  faces.    The  faces 
most  commonly  presented  by  crystals  of  .olivine  are  those 
of -the  rhombic  prism  oo  P  (giving  an  angle  of  little  over 
130°),  the  rhombic   pyramid  P,  the  macropinakoid  oo  P  oo, 
the  brachypinakoid  oo  f  oo,  the  macro-  and  brachydomes 
P  oo  and  £  oo  and  the  basal  plane  o  P.     Olivine  has  two  di- 
/  rections  of  cleavage,  one  parallel  to  the  macro-  the  other  to 
|  the  brachypinakoid,  the  former  being  very  imperfect  and  the 
Ljatter  rather  distinct.      Olivine  varies  in  hardness  from  6 '5 
•^to  7.     Its  fracture  is  conchoidal  and  its  colour  is  not  only 
I  olive  or  bottle-green,  but  at  times  brownish  and  yellowish. 
Where,  therefore,  olivine  occurs  in  rounded  imbedded  gra- 
nules, and  displays  neither  its  characteristic  form  of  crystal 
nor  its  common  green  colour,  its  hardness  and  its  conchoi- 
dal fracture  cause  it  greatly  to  resemble  quartz,  which  also 
occurs   in   rounded  granules  in  certain   rocks  of  eruptive 
>  J?^   origin,  but  a  mistake  of  this  kind  is  only  likely  to  occur  in 
the  hasty  examination  of  hand-specimens  in  the  field.     The 
other  minerals,  however,  with  which  the  granules  are  asso- 
^ciated   usually  give   some   clue   to   their  probable  nature. 
Powdered  olivine  is  easily  decomposed  by  hot  hydrochloric 
.  acid  or  by  sulphuric  acid  ;  separation  of  gelatinous  silica 
occurring  in  either  case.      Before  the  blowpipe  alone,  only 
j  the  highly  ferruginous  varieties  are  fusible,  forming  a  black 


Olivine.  u/ 

magnetic  globule  ;  but  with  borax  the  ordinary  olivine  may  J 
be  fused  to  a  clear  green  bead/  The  foregoing  reactions  ~ 
certainly  serve  to  distinguish  olivine  from  quartz,  but  the 
microscopic  characters  of  the  two  minerals  are  sufficiently 
distinct  to  prevent  any  chance  of  mistake.      The  general 
formula  of  olivine  is  (MgO,   FeO)2   SiO2.      It  sometimes 
contains  some  lime,  alumina,  and  protoxide  of  manganese, 
traces  of  titanic,  phosphoric,  and  chromic  acids,  and  the  pro- 
toxides of  nickel  and  cobalt.    Potash,  soda,  and  small  quan- 
tities of  water  have  also  been  detected  in  olivine.  /  Olivine"} 
frequently  occurs  in  meteorites,  often  forming  a  large  pro-  1 
portion  of  them.     In  some  of  these  meteoric  olivines  traces 
of  arsenious  acid,  fluorine  and  oxide  of  tin  have  been  dis- 
covered.     The    alterations    which    olivine   undergoes   are 
either  due  to  peroxidation  of  the  iron,  or  to  its  removal  by 
water  charged  with  carbonic  acid,  in  which  case  some  of 
the  magnesia  may  also  be  removed.     Changes  of  the  former] 
class  cause  the  mineral  to  assume  a  reddish  or  brownish^ 
colour,  and  sometimes  render  it  iridescent.     This  process 
when  further  advanced  sets  up  a  micaceous  structure  in  the 
mineral.1    Changes  of  the  latter  class,  viz.,  by  water  charged 
with  carbonic  acid,  give  rise  to  the  formation  of  serpentine, 
steatite,  &c.     Many  of  the  Wurtemberg  basalts,  especially 
those  from  H6\ven,  are  little  more  than  serpentine  rocks 
containing  some  magnetite,   since  the  olivine  and  -augite 
which  composed  the  basalt  are  changed  into  serpentine.2 

Under  the  microscope  olivine  appears  almost  colourless") 
when  in  very  thin  sections,  and  of  a  light  greenish  tint  in 
those  of  moderate  thinness.  Olivine  shows  very  feeble 
dichroism,  even  when  thick  sections  are  examined/  In  very 
thin  sections  the  dichroism  is  scarcely  perceptible.  It  is 
doubly  refractive,  polarising  when  in  a  fresh,  undecomposed 
state,  in  moderately  strong  colours,  but  these  are  much  more 
feeble  than  those  displayed  by  quartz.  The  surfaces  of  sec- 

1  Dana,  System  of  Mineralogy,  5th  edition,  p.  258. 

2  Letter  from  Dr.  M.  G.  Reinhold  Fritzgartner. 


1 1 8  The  Rudiments  of  Petrology. 

\tions  of  olivine  are  nearly  always  rough,  since  the  ordinary 
grinding  is  never  capable  of  imparting  a  smooth  polished  face 
to  the  section,  and  these  roughened  surfaces,  which,  when  ex- 
amined under  the  microscope,  present  an  appearance  some- 
what like  that  of  ground  glass,  are  clearly  perceptible  in 
sections  mounted  in  Canada  balsam  and  covered  in  the 
usual  way. 

Lines  of  accretion,  such  as  those  which  form  zones  cor- 
responding with  the  boundaries  of  sections  of  augites  and 
felspars,  have  not  yet  been  observed  in  sections  of  olivine, 
even  when  they  occur  in  rocks  in  conjunction  with  zoned 
crystals  of  those  minerals.  'Olivine  is  not  known  to  occur 
in  the  form  of  very  minute  crystals  or  microliths,  the 
crystals  and  granules  being  usually  sufficiently  large  to  be 
distinguished  without  the  assistance  of  a  lens. 

Crystals  of  olivine  generally  afford  six-  oreight-sided  sec- 
tions, the  angles  of  which  are  often  rounded,  a  fact  regarded 
by  some  observers  as  indicative  of  a  secondary  fusion. 
Quadrangular  sections  are  not  uncommon,  and  unsymmetri- 
cal  forms  frequently  result  from  obliquely-cut  crystals. 
Olivine  is  often  traversed  by  strong  and  irregular  fissures 
which  bear  no  relation  to  the  form  of  the  crystals,  and  some- 
what resemble  the  fissures  visible  in  suddenly  cooled  glass. 
The  alteration  of  olivine  into  serpentine  commences  along 
these  fissures  and  also  along  the  boundaries  of  the  crystal. 
It  appears  under  the  microscope  as  a  finely  fibrous  green 
fringe,  the  fibres  lying  at  right  angles  to  the  surfaces  from 

'  which  they  originate.  As  the  alteration  proceeds,  this  fibrous 
structure  extends  further  inwards  until  the  whole  crystal  is 
converted  into  a  mass  of  interlacing  and  contorted,  or 
radially  disposed,  fibres,  and  no  longer  displays  any  of  the 
optical  characters  which  formerly  belonged  to  it.  rGas 

/pores,  fluid  lacunae,  consisting  generally  of  liquid  carbonic 
acid,  glass  lacunae  and  crystals,  granules  and  microliths  of 
magnetite  are  the  most  common  inclosures  which  occur  in 
olivine.  The  gas  pores  are  often  arranged  in  rows.  The 


Hyper sthene  and  Enstatite.  119 

microliths  frequently  assume  peculiar  forms,  being  sometimes 
zig-zag,  sometimes  claviform  or  acicular.  Inclosures  of 
augite,  felspar,  leucite,  &c.,  are  never  met  with  in  olivine. 

HYPERSTHENE.  v 

Crystalline  system  rhombic.  Usually  occurs,  in  rocks,  in 
a  crystalline  condition  or  in  granules,  but  seldom  in  actual 
crystals.     The  mineral  shows  a  strongly  marked  cleavage 
parallel    to   the   brachypinakoid,   very  imperfect    cleavage 
parallel  to  the  macropinakoid,  and  a  tolerably  perfect  pris- 
matic cleavage.      The  colour  is  black,  greyish,  brownisrA 
green,  or  pinchbeck-brown,  and  presents  in  certain  directionsj 
a  glistening  appearance  or  bronze-like  lustre.  --'  The  chemical 
composition  is  (MgO,  FeO)  SiO2. 

Before  the  blowpipe  it  fuses  to  a  black  enamel,  and,  on) 
charcoal,  to  a  magnetic  mass. 

When  tested  with  a  single  Nicol,  thin  sections  of  hyper- 
sthene  appear  strongly  pleochroic.  The  colour  is  mostly  grey- 
ish-green in  the  direction  of  the  principal  axis  ;  in  the  direc- 
tion of  the  macrodiagonal  it  is  reddish-yellow,  and  in  that  of 
the  brachydiagonal  it  is  hyacinth-red.  Hypersthene  is  not, 
however,  so  strongly  pleochroic  as  hornblende,  but  it  pos- 
sesses this  character  in  so  marked  a  degree  that  it  at  once 
serves  to  distinguish  it  from  diallage,  bronzite,  and  enstatite. 
The  hypersthene  of  Labrador  shows  numerous  imbedded 
lamellae  which  were  regarded  by  Vogelsang  as  possibly 
diallage,  but  A.  von  Lasaulx  thinks  this  improbable,  since 
similar  imbedded  lamellae  have  been  observed  in  diallage 
from  several  localities.  These  plates  have  been  referred  by 
different  authors  sometimes  to  gothite,  to  specular  iron,  to 
brookite,  &c.  Their  true  nature  is,  however,  as  yet  unde- 
termined. Minute  granules  of  magnetite  are  of  frequent] 
occurrence  in  hypersthene,  often  following  one  another  inj 
definite  directions  parallel  to  the  principal  axis./  Boficky 
mentions  the  occurrence  of  minute  plates  of  calcite  imbed- 


I2O  The  Rudiments  of  Petrology. 

ded  in  the  direction  of  principal  cleavage  in  the  hypersthene 
of  a  rock  from  Tellnitzthal.1 

ENSTATITE. 

Crystalline  system  rhombic.  Cleavage  parallel  to  the 
faces  of  the  rhombic  prism  oo  P.  The  cleavage  planes  make 
by  their  intersection  angles,  according  to  Descloizeaux,  of 
88°  and  92°  ;  according  to  Kengott  87°  and  93°.  Enstatite 
also  affords  two  other  directions  of  less  perfect  cleavage,  one 
parallel  to  the  macro-,  the  other  parallel  to  the  brachy- 
pinakoid.  The  cleavage  surfaces  sometimes  have  a  fibrous 
appearance,  and  usually  a  rather  pearly  or  vitreous  lustre, 
while  in  some  varieties  it  is  metalloidal.  In  colour  the 
mineral  ranges  from  greyish-  or  yellowish-white  to  green  and 
brown.  It  occurs  massive  and  lamellar  as  well  as  crystal- 
lised. It  is  almost  infusible  before  the  blowpipe,  the  edges 
of  only  very  thin  splinters  becoming  rounded.  It  is  insolu- 
ble in  hydrochloric  acid.  Enstatite  becomes  altered  to 
schiller-spar  or  bastite,  talc,  &c. 

The  formula  of  enstatite  is  (MgO,  FeO)  SiO2. 

Enstatite  is  strongly  dichroic,  the  greatest  colour  differ- 
ences being  clear  brownish-red  and  pale  sea-green. 

Enstatite  occurs  in  Iherzolite  and  certain  gabbros,  and, 
according  to  Professor  Maskelyne,  some  of  the  diamond- 
bearing  rocks  of  South  Africa  contain  more  or  less  of  this 
mineral. 

BRONZITE. 

This  mineral  crystallises  in  the  rhombic  system.  It  is 
very  difficultly  fusible,  the  edges  only  of  very  thin  splinters 
becoming  rounded  before  the  blowpipe.  It  is  insoluble  in 
acids.  In  its  microscopic  structure  it  is  more  closely  allied 
to  hypersthene  than  to  enstatite.  It  resembles  the  latter  in 
its  strongly  fibrous  structure,  and  the  former  in  the  character 
of  its  numerous  inclosures  which  occur  in  the  form  of 
lamellae,  or,  as  in  the  Kupferberg  bronzite,  of  '  elongated 

1  Petrographische  Studien  an  den  Basalt  gesteinen  Bohmens,  von  Dr. 
E.  Boficky,  p.  16.  Prag,  1874. 


Pyroxene.  1 2 1 

stripes  of  an  acicular  character  set  perfectly  parallel  to  the 
fibrous  structure  and  varying  in  colour  from  dark  honey 
yellow  to  deep  brown.' 1 

Some  bronzite  is  very  feebly  dichroic,  or  scarcely  di- 
chroic  at  all.  In  this  respect  bronzite  differs  both  from  ensta- 
tite  and  hypersthene-  The  directions  of  cleavage  in  bronzite 
are,  parallel  to  the  brachypinakoid,  highly  perfect ;  parallel 
to  the  right  rhombic  prism,  imperfect  ;  parallel  to  the 
macropinakoid,  most  imperfect.  The  formula  of  bronzite  is 
(MgO,  FeO)  SiO2.  Some  analyses  show  the  presence  of  lime 
and  alumina. 

PYROXENE  GROUP. 

All  the  minerals  of  this  group  crystallise  in  the  mono^ 
clinic  system  and  are  bi- silicates  of  different  protoxides, 
having  the  general  formula  RO,  SiO2,  the  protoxides  often 
being  converted  into  sesquioxides.  The  protoxides  are 
lime,  magnesia,  sometimes  potash  and  soda,  and  the  protox- 
ides of  iron  and  manganese.  The  different  members  of 
this  group  vary  more  or  less  in  composition,  a  fact  due  to 
the  respectively  isomorphous  character  of  their  protoxides 
and  sesquioxides.  The  most  important  difference  in  the 
chemical  composition  of  the  pyroxenes  and  the  amphiboles, 
both  of  which  groups  have  the  same  general  formula,  lies  in 
the  fact  that  in  the  former  lime  is  in  all  the  varieties  of  the 
group  an  important  constituent,  while  in  some  of  the  varie- 
ties of  the  latter  group  it  is  either  totally  absent  or  occurs 
only  in  very  small  quantity. 

Chemically,  the  pyroxenes  may  be  divided  into  those 
which  are  poor  in,  and  those  containing  from  three  to  over 
nine  per  cent,  of,  alumina.  The  diallages  in  this  respect 
range  between  the  two  divisions.  Some  of  the  so-called 
diallages  (metalloidal  diallage),  belong  rather  to  enstatite 
than  to  pyroxene,  since  the  crystallisation  is  rhombic.  Des- 
cloiseaux  regards  the  diallage  occurring  in  the  Cornish 
serpentines  as  probably  a  form  of  enstatite. 2 

1  Mik.  Beschaff.  d.  Min.  u.  Gest.,  Zirkel,  p.  187. 

2  System  of  Mineralogy,  J.  D.  Dana,  5th  edition,  1871,  p.  209.    In 


122  The  Rudiments  of  Petrology. 

Augite  crystals  frequently  occur  twinned,  the  plane  of 

!  twinning  being  parallel  to  the  orthopinakoid. 

The  angle  formed  by  the  oblique  rhombic  prism  in 
augite  is  87°  5'.  The  corresponding  angle  in  hornblende 
crystals  is  124°  30'.  This  great  discrepancy  in  their  angular 
measurements  serves  to  distinguish  the  minerals  of  the 
one  species  from  those  of  the  other,  and  even  under  the 
microscope,  when  the  planes  of  sections  often  lie  obliquely 
to  the  principal  crystallographic  axis,  a  rudely  approximate 
measurement  frequently  enables  the  observer  to  discriminate 
between  hornblende  and  augite.  Another  means  of  dis- 

.  languishing  between  these  two  minerals  is  afforded  by  the 
strong  dichroism  which  usually  characterises  hornblende,  and 
the  very  feeble  dichroism  presented  by  sections  of  augite 
under  the  microscope  ;  the  latter  giving  only  a  succes- 
sion of  slightly  different  tints,  while  in  hornblende  an  actual 

.difference  in  colour  is  to  be  observed. 

It  sometimes  happens,  however,  that  very  thin  sections 
of  hornblende  exhibit  only  feeble  dichroism,  and  that  thick 
sections  of  augite  may  show  this  character  in  a  rather 
marked  manner.  Still  the  test  of  dichroism  may,  as  a  rule, 
be  accepted  with  considerable  safety.  This  character  is  best 
shown,  in  thin  sections,  by  removing  the  analyser  from  the 
microscope,  and  by  rotating  the  polariser. 

Such  dichroism  as  augite  sections  may  exhibit  is  always, 
according  to  Mr.  Allport's  observations,1  connected  with  a 
purple  tinge,  and  he  has  noted  this  fact  in  augites  occurring 
in  rocks  of  very  different  ages  and  derived  from  widely 
separated  localities.  He  also  states  that  dichroism  is  more 
strongly  elicited  from  those  augite  crystals  which  by  ordi- 
nary illumination  exhibit  a  variation  in  colour,  i.e.  when 
one  end  or  side  of  a  crystal  appears  purplish-brown  and 

Descloiseaux's  Mamiel de  Mineralogie,  t.  i.,  1862,  there  is,  however,  no 
mention  of  this  ;  the  Cornish  serpentines  being  there,  and  at  that  time, 
cited  as  containing  diallage. 

1  '  On  the  Microscopic  Structure  and  Composition  of  British  Car- 
boniferous Dolerites,'  S.  Allport,  Q.  y.  G.  S.,  vol.  xxx.  p.  536. 


Augite.  123 

shades  off  into  yellowish-brown  towards  the  opposite  end  or 
side. 

By  ordinary  transmitted   illumination   thin  sections   of! 
augite   appear   of  a  greenish    or  yellowish-brown   colour.  ; 
Wedding  has  noted  the  occurrence  of  greenish  and  brown- 
ish layers,  sharply  defined,  in  the  augites  of  the  Vesuvian 
lavas,  and  concludes  that  the  latter  colour  is  not  due  to  oxi- 
dation of  ferrous  compounds  constituting  the  greenish  por- 
tions of  the  crystals. 

Similar  bands,  frequently  well  marked  and  of  variegated 
colour,  are  often  to  be  seen  by  polarised  light  in  sections  of 
augite  and  represent  twin  lamellae  /which  lie  parallel  to  the 
orthopinakoid.  Even  by  ordinary  transmitted  light  they 
may  sometimes  be  recognised  as  exceedingly  delicate  striae. 

Sections  of  hornblende  and  augite,  when  cut  at  right 
angles  to  the  principal  axis,  may  usually  be  distinguished  by 
the  augite  giving  eight-sided  sections,  as  a  rule  ;  while  those 
of  hornblende  are  generally  six-sided. 

In  some  sections  of  augite  crystals  striae  are  visible 
under  the  microscope,  which  correspond  with  the  boundaries 
of  the  section,  or,  in  other  words,  with  the  external  form  of 
the  crystal.  These  lines  lie  one  within  the  other,  constituting 
zones  of  variable  width,  and  are  often  rendered  still  more 
apparent  by  granules  of  magnetite,  microliths,  and  small 
cavities,  which  follow  or  coincide  with  these  lines  in  a  re- 
markably regular  manner.  In  polarised  light  the  concentric 
bands  exhibit  different  tints  and  appear  more  strongly 
marked.  They  no  doubt  represent  lines  of  accretion. 

The  inclosures  in  augite  crystals  consist,  so  far  as  they  \ 
have  yet  been  observed,  of  acicular  microliths  of  augite 
itself.  Triclinic  felspars  and  leucite  also  occur  in  minute 
crystals  imbedded  in  augite,  and  the  sides  of  augite  crystals 
are  often  penetrated  by  crystals  of  apatite.  Magnetic  and 
titan iferous  iron  are  of  frequent  occurrence  in  augite,  some- 
times almost  entirely  replacing  it.  Furthermore,  augite 
often  contains  inclosures  of  amorphous  glass,  sometimes 


1 24  The  Rudiments  of  Petrology. 

(spherical,  sometimes  irregular  in  form,  and  occasionally  cavi- 
Jjies  containing  fluid  have  been  observed.  In  some  excep- 
tional cases  the  augites  of  certain  basalts  are  merely  repre- 
sented by  a  thin  line  or  border  of  augitic  substance,  the 
interior  of  the  crystal  being  filled  with  the  admixture  of 
minerals  which  constitutes  the  ground-mass  of  the  rock. 
(The  nature  of  the  minerals,  &c.,  inclosed  in  augite  crystals 
idepends  of  course  very  materially  upon  the  mineral  compe- 
tition of  the  rocks  in  which  the  augites  occur. 

This  mineral  does  not  merely  occur  in  rocks  in  the  form 
of  well-defined  crystals,  but  also  as  acicukr  microliths  which 
are  generally  greenish  or  yellowish-brown,  unless  they  are 
exceedingly  minute,  in  which  case  they  are  almost  colourless. 

These  microliths  vary  considerably  in  form,  some  being 
straight,  some  curved,  while  occasionally  they  are  forked  at 
the  ends,  or  assume  club-like  forms.  Sometimes  they  lie 
independently  scattered  through  the  matrix  of  the  rock,  at 
others  they  are  clustered  together,  especially  around  the 
margins  of  crystals. 

Diallage,  which  is  a  common  constituent  of  some  rocks, 
is  usually  regarded  as  a  variety  of  augite.  It  exhibits  a 
highly  perfect  cleavage  parallel  to  the  clinodiagonal,  thus 
differing  from  augite,  in  which  the  corresponding  cleavage  is 
imperfect.  Both  of  these  minerals  also  possess  a  perfect,  or 
tolerably  perfect,  cleavage  parallel  to  the  faces  of  the  oblique 
rhombic  prism  (PI.  II.  p.  173).- 

Diallage  resembles  augite  in  displaying  very  weak 
dichroism,  so  weak  that  it  gives  rise  to  no  marked  dif- 
ference of  colour,  and  may  thus  be  distinguished  from  horn- 
blende and  hypersthene.  Thin  plates  of  diallage  when  ex- 
amined microscopically  are  often  seen  to  include  numerous 
little  tabular  crystals  and  acicular  microliths.  The  tabular 
crystals  mostly  run  in  definite  planes  parallel  to  the  ortho- 
and  to  the  clinopinakoids  of  the  diallage  and  also  to  a  third 
plane  which  lies  obliquely  to  that  of  the  orthopinakoid. 
'  Sections  taken  parallel  either  to  the  ortho-  or  clinopinakoid 
always  exhibit  the  broad  sides  of  those  little  imbedded 


Diallage.  125 

crystals  which  are  conformable  to  the  face  under  examina- 
tion, together  with  the  line-like  transverse  sections  of  similar 
crystals  which  lie  with  their  broad  sides  parallel  to  the  other 
face/1  The  acicular  microliths  in  some  diallages  also  follow 
definite  directions  and  appear  to  cross  one  another  obliquely, 
forming  a  somewhat  lozenge-shaped  net  work.  Fluid  lacunae 
have  likewise  been  observed  in  the  diallage  of  a  Silesian 
gabbro.  Diallage  sections  usually  exhibit  a  pale  greenish  or 
brownish-yellow  tint  by  ordinary  transmitted  light. 

Asbestus  is  in  some  cases  a  fibrous  form  of  augite,  but 
most  asbestus  is  hornblendic  in  its  affinities. 

Breislackite  is  a  capillary  or  woolly  form  of  pyroxene, 
occurring  in  the  lavas  of  Vesuvius  and  Capo  Di  Bove. 
Although  definitely  known  to  be  a  pyroxene,  the  precise 
species  to  which  it  belongs  has  not  yet  been  determined. 

Diallagic  Augite. — A  form  of  pyroxene  intermediate  in 
character  between  augite  and  diallage  has  been  noticed  by 
Professor  E.  Boficky,  of  the  university  of  Prag,  in  his 
'  Petrographische  Studien  an  den  Melaphyr  Gesteinen 
Bohmens,'  p.  19.  He  describes  it  as  an  augite  whose  sec- 
tions may  be  distinguished  from  ordinary  augite  by  the 
occurrence  of  straight  and  parallel  fissures  or  striae  which,  in 
longitudinal  sections  of  the  crystals,  cross  the  coarser  cleav- 
age planes  at  angles  of  from  70°  to  90°.  Professor  Boficky 
considers  these  lines  to  represent  the  edges  of  sections  of 
-  delicate  lamellae  which  he  regards  as  twin-like  intergrowths, 
most  probably  lying  parallel  to  the  basal  planes  of  the  crys- 
tals. He  states  that  in  the  melaphyres  of  Neudorf  near 
Lomnitz  the  diallagic  augite  sections  are  broad,  irregularly 
bounded,  and  contain  numerous  bubbles  and  stone  lacunae 
or  'stone  cavities'  (Schlackenkornchen]  which  are  often  ranged 
in  lines,  either  parallel  to  the  fissures  or  actually  along  them. 
Such  fissures  or  striae  often  occur  in  one  part  only  of  an 
individual  crystal.  The  mineral  is  not  dichroic,  and  polarises 
in  strong  colours,  the  crystal  sections  sometimes  presenting 

1  Mikroskop.  Beschaff.  der  Min.  und  Ge.st.,  Zirkel,  p.  181. 


126 


The  Rudiments  of  Petrology. 


iris- coloured  margins.  He  has  noted  their  occurrence  in 
the  melaphyres  of  Hofensko,  Lomnitz,  Neudorf  near  Lomnitz, 
and  Zdiretz  in  Bohemia. 

ALTERED  CONDITIONS  OF  PYROXENE. 

It  is  important  that  the  microscopist  should  have  some 
knowledge  of  the  alterations  which  pyroxene  undergoes,  in 
order  rightly  to  understand  the  nature  of  the  pseudomorphs 
after  the  different  members  of  this  group  which  so  frequently 
occur  in  eruptive  rocks. 

The  simplest  kind  of  alteration  which  pyroxenic  minerals 
experience  is  hydration.  It  is  frequently  accompanied  by 
a  loss  of  silica.  The  lime  and  iron  contained  in  these 
minerals  also  undergoes  considerable  diminution,  or  is  totally 
removed  by  water  charged  with  carbonic  acid  or  holding 
carbonates  in  solution. 

The  following  are  some  of  the  results  of  the  alteration  of 
pyroxene. 

TABLE  SHOWING  THE  APPROXIMATE   CHEMICAL  COMPOSI- 
TION   OF    THE  VARIOUS    MINERALS  WHICH    RESULT   FROM 

THE  ALTERATION  OF  PYROXENE. 


Si 

a 

& 

*e 

Fe 

Mn 

Mg 

Ca 

Na 

* 

Ti 

H 

Augite  . 
Pycrophyll    . 
Pyrallolite     . 
Schiller-spar  . 

51 

494 
43 

3 

2 

4 
2 



- 

6 
6 

ii 

3 

i 

36 

24 



- 

- 

10 

12! 

Rammelsberg 
Rose 
Runeberg 
Kohler 

Epidote 

46 

el 

_ 

JO 

8 

— 

12* 

9 

— 

— 

— 

5 

Streng 

Mica      . 

43 

15" 





23* 

— 

io| 

i 

i 

5 

— 

Kjerulf 

Uralite  . 

49 

I 





— 

12 

"4 

— 

— 

i 

Rath 

Glauconite    . 

5J4 

7 





21 

_ 

6 

2 

6 

— 

6| 

Delesse 

Serpentine     . 

41 

2 





2 

— 

42 

— 



— 

— 

13 

Scheerer 

Steatite 



— 



*4 

— 

31 

— 



— 

—  - 

5 

Richter 

64 

I 





.  — 

— 

28 

— 



— 

— 

Tengstrom 

Palagonite     . 

42 

— 

16 

— 



7 

7 

2 

— 

— 

12^ 

Waltershausen 

Hematite 

— 



— 

100 



— 

— 

— 



— 

— 



— 

Limonite 

24 



— 

— 

— 

— 

— 



— 

— 

16 

Ulmann 

Magnetite     . 
Titaniferous  ) 
Magnetite    j 

— 

- 

- 

69 

22 

68 

31 
30 

2 

- 

- 

- 

- 

2 

- 

Knop 

Michaelson 

The  list  is  headed  by  a  typical  analysis  of  unaltered 
augite  from  Schima  in   Bohemia.     The   decimals  in  the 


Amphibole.  127 

original  analyses  have  been  omitted,  and  the  fractions  rather 
differently  distributed,  in  order  to  simplify  the  columns  for 
reference. 

In  addition  to  these  analyses  may  be  cited  one  of  a 
highly  siliceous  pseudomorph  after  pyroxene,  by  Rammels- 
berg,  consisting  of  85  per  cent,  silica,  \\  alumina,  2  protox- 
ide of  iron,  2  magnesia,  2\  lime,  and  6  water. 

Serpentine,  steatite,  and  limonite  are  probably  the  most 
common  of  these  alteration-products  in  British  eruptive 
rocks  Epidote  seldom  gives  direct  evidence  of  its  deriva- 
tion from  pyroxenic  minerals,  since  it  generally  occurs  in 
characteristic  crystals  or  radiating  tufts  along  minute  lines  of 
fracture,  and  not  in  pseudomorphs  bounded  by  the  outlines 
of  pyroxenic  forms.  Nevertheless,  it  does  not  follow  that  it 
has  not,  in  these  cases,  resulted  from  the  decomposition  of 
pyroxene.  Pseudomorphs  after  pyroxene  of  quartz,  opal,  and 
calcite  are  also  of  occasional  occurrence,  but  these  are  more 
probably  due  to  subsequent  infiltrations,  than  to  a  partial' 
removal  of  the  basic  constituents,  or,  in  the  case  of  opal,  to 
the  hydration  of  a  residuum  of  silica.  The  usual  mode  of 
deposition  of  hydrous  silica,  opal  hyalite,  &c.,  in  vesicles  and 
cavities,  and  on  the  surfaces  of  joint  planes  in  eruptive  rocks, 
tends  to  support  this  view.1 

AMPHIBOLE  GROUP. 

The  minerals  of  this  group  closely  resemble  pyroxene  in 
chemical  composition,  while  they  also  crystallise  in  the  same 
system  (monoclinic).  They  differ,  however,  as  already 
pointed  out,  in  the  angular  measurements  of  the  oblique 
rhombic  prism,  which  in  hornblende  is  124°  30'  and  in 
augite  from  87°  5'  up  to  92°  55'.  They  are  all  bi-silicates 

1  According  to  Mitscherlich,  Berthier,  and  G.  Rose,  tremolite  and 
actinolite  (both  varieties  of  amphibole)  yield,  when  fused  in  a  porcelain 
furnace,  forms  similar  to  those  of  pyroxene.  Crystals  of  augite  are  of 
common  occurrence  in  blast  furnace  slags,  sometimes  even  associated 
with  hornblende  crystals,  but  the  latter  are  less  frequently  met  with ; 
and  it  is  stated  that  Mitscherlich  and  Berthier,  although  able  to  produce 
artificial  crystals  of  augite,  failed  to  procure  any  of  hornblende. 


128  The  Rudiments  of  Petrology. 

of  protoxides  and  sesquioxides,  the  former  being  lime,  mag- 
nesia, soda,  potash,  and  the  protoxides  of  iron  and  manga- 
nese, while  the  latter  are  represented  by  alumina  and  the 
peroxides  of  iron  and  manganese.  Crystals  of  amphibole 
differ  from  those  of  pyroxene,  not  merely  in  the  -angular 
measurements  of  their  oblique  rhombic  prisms,  but  also  in 
the  angles  at  which  their  cleavage  planes  intersect.  In  both 
groups  the  relation  of  the  cleavages  to  the  respective  faces 
of  the  crystals  is  the  same,  but  they  differ  in  their  respective 
facilities  ;  the  cleavage  parallel  to  the  faces  of  the  oblique 
hombic  prism  in  hornblende  being  more  perfect  and  more 
trongly  marked  than  the  corresponding  cleavage  in  augite, 
while  the  cleavages  parallel  to  the  pinakoids  are  on  the  other 
hand  less  strongly  marked  in  hornblende  than  in  augite. 
Furthermore,  the  discrepancy  in  the  angles  of  the  two  respec- 
tive oblique  rhombic  prisms  begets  a  corresponding  discre- 
pancy in  the  angles  at  which  the  prismatic  cleavages  of  the 
two  different  species  intersect.  This  circumstance  is  of  con- 
siderable value  to  the  mineralogist,  since  it  is  often  difficult 
or  impossible  to  measure  the  angles  of  the  actual  crystallo- 
graphic  faces,  but  it  is  generally  possible  to  measure  the 
angles  of  cleavage,  and  since  these  cleavages  respectively 
coincide  with  the  plane  co  P,  the  results  deduced  from 
cleavage  are  as  good  as  those  derived  from  the  actual  faces 
of  the  crystals. 

The  crystals  of  minerals  belonging  to  the  amphibole  group 
usually  exhibit  a  fine  longitudinal  striation. 

The  very  feeble  dichroism  of  augite  and  the  strong  di- 
chroism  of  hornblende  has  already  been  mentioned,  and, 
although  exceptional  cases  may  occur,  it  must  nevertheless 
be  regarded  as  a  most  valuable  test,  especially  in  the  micro- 
scopic examination  of  rocks  which  contain  minerals  pertain- 
ing to  one  or  other  of  these  groups. 

The  amphiboles,  like  the  pyroxenes,  may  be  divided  into 
two  sub-groups,  viz.,  those  which  contain  little  or  no 
alumina,  and  those  which  are  rich  in  that  base  (sometimes 


Hornblende.  1 29 

containing  over  15  per  cent.)  The  similarity  in  the  chemi- 
cal composition  of  amphibole  and  pyroxene  begets  a  simi- 
larity in  the  minerals  which  result  from  their  alteration,  so 
that  the  alteration-products  tabulated  at  the  end  of  the 
pyroxene  section  may  be  taken  as  fairly  representative  of 
the  changes  which  amphibole  undergoes. 

In  the  microscopic  examination  of  thin  sections  of  hornj 
blende,  the  forms  are  often  so  irregular,  that  it  is  difficult  to 
arrive  at  any  sound  deductions  from  their  contours. 

Colour  also  affords  no  safe  means  of  discriminating 
between  pyroxene  and  amphibole,  since  the  members  of  both 
groups  exhibit  greenish  and  brownish  tints. /The  augites  and  j 
hornblendes  which  occur  in  basalt  are  mostly  brownish  in 
colour.  The  hornblende  in  syenite  is  also  generally  brown, 
but  that  which  occurs  in  phonolite  is  mostly  of  a  greenish 
tint  while  the  augite  in  leucite-lavas  is,  as  a  rule,  also  green. 

The  most  important  microscopic  characteristics  of 
common  hornblende  may  be  summed  up  in  the  following 
manner.  Transverse  sections  of  crystals  show  two  sharply' 
denned  sets  of  cleavage  planes,  each  set  corresponding  with 
the  opposite  and  alternate  faces  of  the  oblique  rhombic]'; 
prism/and  intersecting  at  an  angle  of  124°  30'  (when  the 
section  is  at  right  angles  to  the  principal  crystallographic 
axis).  In  sections  taken  parallel  to  the  chief  axis  of  then 
crystal  a  fine  longitudinal  striation,  indicative  of  a  fibrous 
structure,  is  often  to  be  observed. 

Furthermore,  the  dichroism  of  hornblende  is  very  strong, 
although  the  clear  green  varieties,  as  pointed  out  by  Zirkel, 
show  only  very  feeble  dichroism  and  might  be  mistaken  for 
augite.1 

Some  of  the  small  crystals,  such  as  those  often  occurring 
in  phonolites,  appear  to  be  made  up  of  fine  parallel  rods 
or  elongated  microliths,  the  margins  being  frequently  very 
irregular. 

Microliths  of  hornblende  are  of  common  occurrence  in 

1  Zirkel,  Mik.  Beschajf.  d.  Min.  u.  Gesteine,  p.  169. 
K 


1 30  The  Rudiments  of  Petrology. 

I  eruptive  and  metamorphosed  rocks ;  when  very  minute  they 
are  often  colourless.  They  seldom  present  any  recognisable 
crystalline  form,  but  frequently  exhibit  longitudinal  fibrous 
structure,  often  appearing  to  be  frayed  out  at  the  ends. 

Zone-like  bands  of  accretion,  similar  to  those  which  are 
sometimes  visible  in  sections  of  augite,  also  occur  in  horn- 
blende sections,  t  Magnetite,  apatite,  nepheline,  biotite, 

j  quartz,  &c.,  occur,  enclosed  in  hornblende  crystals  ;  glass 
lacunae  are  also  frequently  met  with  in  the  hornblendes  of 

Ltrachytes,  phonolites,  pitchstones,  &c. 

The  hornblendes  of  syenites,  diorites,  &c,  contain  as  a 
rule  fewer  microscopic  inclosures  than  those  of  volcanic 
rocks,  such  as  trachytes,  phonolites,  basalts,  &c. 

Crystals  of  hornblende  frequently  exhibit  a  dark  granular 
^border  of  magnetite,  and  at  times  the  latter  mineral  has  been 
observed  greatly  to  dominate  over  the  hornblende,  the  horn- 
blendic  matter  merely  appearing  as  little  interstitial  specks 
between  the  magnetite  granules.  In  such  crystals,  one  may 
almost  say  pseudomorphs,  the  outlines  of  the  sections  still 
clearly  denote  the  form  of  hornblende ;  and  it  seems,  as  sug- 
gested by  Zirkel,  that  in  these  cases  even  a  very  small  pro- 
portion of  hornblende,  in  spite  of  an  almost  overwhelming 
percentage  of  magnetite,  is  capable  of  asserting  its  crystal- 
line form,  just  as  in  the  well  known  Fontainebleau  sandstone 
a  trivial  proportion  of  calcite  develops  rhombohedral  crystals 
which  contain  as  much  as  65  per  cent,  of  sand.1  In  this 
latter  case  the  sand,  as  stated  by  Delesse,  was  formed  prior 
to  the  crystallisation  of  the  calcite ;  in  other  words  the  silica 
and  the  carbonate  of  lime  did  not  crystallise  at  the  same 
time.  It  is  therefore  probable  that  the  granular  magnetite 

j  was  also  developed  prior  to  the  crystallisation  of  the  horn- 

Lblende,  and  was  simply  taken  up  by  it. 

The  minerals  of  the  amphibole  group  frequently  show  a 
tendency  to  develope  long  blade-like  crystals.  One  of  the 

1  Delesse, '  Recheiches  sur  les  Pseudomorphoses, '  Annales  des  Mines, 
t.  xvi.  1859.  (Separately  printed  extract,  p.  35.) 


Hornblende.  131 

principal  varieties,  actinolite,  shows  this  tendency  in  a  very 
marked  manner,  the  crystals  arranging  themselves  in  radiate 
groups.  Actinolite  is  a  magnesia-lime-iron  amphibole ;  it  is 
usually  of  a  dark  green  colour.  Under  the  microscope  the 
crystals  mostly  appear  pale  green  by  ordinary  transmitted 
light,  and  are  often  traversed  by  numerous  transverse 
fractures,  frequently  accompanied  by  displacement  of  the 
crystal  on  either  side  of  these  cracks.  Some  of  the  long 
radiating  crystals  in  a  serpentinous  rock  from  Cannaver 
Island,  Galway,  Ireland,  display  magnificent  variegations  of 
colour  under  polarised  light. 

Tremolite  is  a  magnesia-lime  amphibole  (CaO,  MgO) 
Si02.  In  colour  it  is  generally  white  or  greyish.  Nephrite 
or  jade  is  in  part  a  tough  compact  fine-grained  tremolite, 
having  a  tinge  of  green  or  blue,  and  breaking  with  a  splintery 
fracture  and  glistening  lustre.  It  usually  occurs  associated 
with  talcose  or  magnesian  rocks.'  ! 

Asbestus  or  amianthus  is  a  fibrous  variety  of  pyroxene 
occurring  in  white  silky  fibres,  which  are  matted  together, 
but  are  easily  separable.  Byssolite  is  more  compact  in 
aggregation,  the  fibres  are  coarser  as  a  rule  and  are  not  easily- 
separated,  the  structure  more  resembling  that  of  wood<,  while 
the  colour  is  usually  dark  green  or  greenish  grey.  It  may 
be  regarded  as  an  iron-manganese  amphibole, 

All  these  varieties,  viz.,  actinolite,  tremolite,  jade,  as- 
bestus,  and  byssolite  belong  to  the  non-aluminous,  or  almost 
non-aluminous,  sub-group  of  amphibole. 

Ordinary  hornblende,  its  variety  pargasite,-  and  smarag- 
dite,  which  is  a  foliated  grass-green  form  of  hornblende, 
somewhat  resembling  diallage  in  appearance,  are  varieties  of 
the  aluminous  sub-group  of  amphibole. 

Hornblende  is  frequently  twinned  along  a  plane  parallel 
to  the  orthopinakoid.  This  gives  rise  to  a  difference  in  the 
terminations  of  the  crystals,  one  end  exhibiting  four  faces 
(hemidomes),  and  the  other  two  faces  (basal  planes). 

1  Dana,  System  of  Mineralogy,  5th  edition,  p.  233. 
K  2 


132  The  Rudiments  of  Petrology. 

Both  hornblende  and  augite  sometimes  occur  together  in 
the  same  rock  ;  but  as  a  rule  the  former  mineral  is  found  in 
those  rocks  which  contain  a  large  percentage  of  silica,  the 
associated  minerals  being  usually  quartz  and  orthoclase, 
while  augite  is  generally  found  in  rocks  of  a  basic  character 
containing  triclinic  felspars,  and  with  little  or  no  free  silica. 
Augite  and  its  variety,  diallage,  sometimes  occur  together  in 
the  same  rock,  and  in  such  cases  the  petrologist  often  has 
difficulty  in  the  precise  determination  of  the  pyroxenic 
constituents  ;  the  diallagoid  augite  *  of  Boricky  sufficiently 
evinces,  merely  from  the  name  which  has  been  given  to  it, 
the  intermediate  character  which  such  pyroxenic  minerals 
may  sometimes  possess. 

MICA  GROUP. 

The  minerals  of  this  group,  most  commonly  occurring 
as  constituents  of  rocks,  crystallise  either  in  the  hexagonal 
or  in  the  rhombic  system.  They  have  a  highly  perfect 
cleavage  parallel  to  the  basal  plane,  and  the  thin  laminae 
procured  by  cleavage  are  not  merely  flexible  but  also  elastic, 
springing  back,  when  bent,  into  their  original  position.  The 
hexagonal  species  are  uniaxial,  the  rhombic  biaxial.  The 
micas  mostly  have  a  pearly  or  sub-metallic  lustre.  Their 
hardness  is  very  variable,  some  of  them  ranging  as  low  as  i, 
while  one  species,  ottrelite,  is  sufficiently  hard  to  scratch  glass. 

In  chemical  composition  the  micas  vary  considerably, 
and  cannot  well  be  represented  by  a  general  formula.  They 
are  silicates  of  alumina,  with  silicates  of  potash,  magnesia, 
or  lithia,  and  protoxides  of  iron  and  manganese.  Some 
species  are  hydrous,  but  this  condition  implies  alteration  in 
most  instances.  It  is  now  considered  probable  by  some 
mineralogists  that  isomorphous  mixtures  of  the  different 
species  may  occur  amongst  the  micas,  just  as  they  do 
among  the  felspars. 

1  Diallagahnlicher  Atigit.  Boricky,  Petrographische  Studien  an  den 
Melaphyrgcsteinen  Bohmens,  p.  19.  Prag,  1876. 


Rhombic  Micas.  133 

RHOMBIC  MICA  SECTION. 

A  Muscovite.  —  A  potash  mica,  optically  biaxial,  crystallising 
in  the  rhombic  system,  and  usually  occurring  in  rhombic  or 
six-sided  tabular  crystals  ;  the  lateral  faces  are  sometimes 
those  of  pyramids,  sometimes  of  the  rhombic  prism,  afford- 
ing in  the  latter  case  an  angle  of  about  120°,  the  opposite 
acute  angles  often  being  truncated  by  faces  of  the  brachy- 
pinakoid.  In  many  rocks  the  crystals  are  but  poorly  deve- 
loped, or  only  represented  by  irregularly-shaped  scales, 
which  occasionally,  but  rarely,  exhibit  a  slight  curvature. 
Cleavage  basal  and  very  perfect.  Hardness  =  2  to  3.  Colour, 
mostly  silvery  white  ;  seldom,  but  occasionally,  dark  brown 
or  black. 

The  percentage  chemical  composition  of  muscovite  may 
be  regarded  typically  as  :  —  • 


Before  the  blow-pipe  it  whitens  and  fuses  on  thin  edges 
to  a  grey  or  yellow  glass.  Muscovite  is  not  decomposed  by 
sulphuric  or  hydrochloric  acids. 

Under  the  microscope  sections  of  muscovite  appear 
transparent,  and  exhibit  clear  colours.  Plates,  viewed  at  right 
angles  to  the  basal  plane,  show  tolerably  strong  chromatic 
polarisation,  in  this  respect  differing  from  the  uniaxial  micas  * 
which,  under  similar  conditions,  become  dark'  between 
crossed  Nicols.  When  tested  for  dichroism  it  shows  but 
little  change  of  actual  colour  ;  as  a  rule  merely  displaying  a 
change  from  light  to  dark  shades  of  the  same  colour.  Two 
systems  of  striae  are  often  visible  under  the  microscope  on 
the  surfaces  of  thin  plates,  the  one  set  running  parallel  to 
oo  P  and  oo  P  co  ,  the  other  less  perfect  following  oo  £  3  and 
oo  P  oo  .  These  directions  of  striation  bear  a  definite  relation 
to  the  optical  axes.  Fluid  inclosures  occur  in  some  micas,' 
but  they  contain  no  bubbles,  and,  as  suggested  by  Dr.  A. 
von  Lasaulx,  are  doubtless  merely  infiltrations  which  have 


134  The  Rudiments  of  Petrology. 

(occurred  along  cleavage  planes.  Tourmaline,  apatite,  garnet, 
/quartz,  magnetite,  and  undetermined  microliths  have  been 
,observed  as  inclosures  in  muscovite;  but,  as  a  rule,  the 
•mineral  contains  few  foreign  matters.  Newton's  rings  are 
I  often  visible  between  the  laminae.  Small  crystals  and,  at 
j  times,  interlaminations  of  biotite  occur  in  muscovite. 

Fuchsite  is  a  variety  of  muscovite  containing  about  4 
per  cent,  of  chromic  acid. 

^Lepidolite  (lithia  mica)  corresponds  cry  stall  ©graphically 
and  physically  with  muscovite,  for  which  it  is  frequently  a 
substitute  in  granites.  It  usually  occurs  in  fine  scaly  or 
granular  aggregates,  rather  than  definite  crystals.  The 
colour  is  generally  violet,  rose-red,  or  violet-grey,  and  occa- 
sionally white. 

In  chemical  composition  lepidolite  may  be  regarded  as 
muscovite,  in  which  the  potash  is  partially  replaced  by 
lithia.  An  analysis  of  lepidolite  from  Rozena  in  Moravia 
gave  silica  =  50-32,  alumina  =  28-54,  peroxide  of  iron.=  0-73, 
magnesia  =  0*51,  lime=roi,  lithia  =  o-7o,  fluoride  of 
lithium  =  0-70,  fluoride  of  potassium  =  12*06,  fluoride  of 
sodium=i77,  rubidia  =  o'24,  csesia  =  traces,  water  =  3'i2. 
Lepidolites  from  Uto  in  Sweden  have  yielded  over  5  -5  per 
cent,  of  lithia.  Before  the  blowpipe  lepidolite  colours  the 
flame  purple-red.  After  fusion  before  the  blowpipe,  it  is 
completely  decomposed  by  acids ;  but  otherwise  it  is  only 
imperfectly  soluble. 

Damourite  and  Seriate  are  hydrous  potash  micas  usually 
occurring  in  scaly  aggregates,  but  their  crystallographic 
system  has  not  yet  been  determined.  Sericite  occurs  in 
undulating  scales  which  have  a  fibrous  structure.  These 
wavy  scales  often  run  through  the  schistose  rocks  in  which 
they  occur  in  tolerably  parallel  directions  ;  at  other  times 
they  anastomose  or  form  a  mesh-work.  The  fibrous  struc- 
ture distinguishes  it  from  mica.  Each  fibre  has  an  in- 
dividual polarisation,  a  character  which  is  very  constant.  It 
may  be  distinguished  from  chlorite  by  the  absence  or  feeble- 


Hexagonal  Micas.  135 

jjess  of  dichroism.  By  ordinary  transmitted  light  thin 
sections  of  sericite  are  colourless,  yellowish,  or  greenish,  but 
never  of  so  strong  a  green  colour  as  chlorite. 1 

Paragonite  is  a  hydrous  soda  mica. 

Margarodite,  crystallographically  and  optically,  is  almost 
identical  with  muscovite,  and  results  from  the  hydration  of 
that  mineral.  Chemically  it  approximates  to  damourite. 
The  colour  is  generally  silvery  white,  and  it  has  a  more 
pearly  lustre  than  muscovite. 

Phlogopite  crystallises  in  the  same  system,  and  has  the 
same  cleavage  as  muscovite.  The  divergence  of  the  optical 
axes  is  from  5°  to  20°.  The  colour  is  usually  brown  or 
copper-red,  sometimes  white.  Thin  laminae  often  show  a 
stellate  figure  when  a  candle  flame  is  looked  at  through 
them ;  this  asterismus  is  due  to  the  presence  of  included 
microliths  or  small  crystals,  which  follow  three  definite 
directions.  The  chemical  composition  is  essentially  silicate 
of  alumina,  magnesia,  potash,  and  frequently  soda.  Mag- 
nesia is  sometimes  present  from  20  to  30  per  cent  It  is  a 
mica  which  mostly  occurs  in  crystalline  limestones,  dolomites, 
and  serpentines.  The  dichroism  of  this  mineral  is  strong. 
Its  microscopic  character  is  not,  however,  sufficiently  well 
marked  to  enable  it  easily  to  be  distinguished  by  this  means 
from  muscovite  or  lepidolite,  chemical  analysis  being  the 
only  trustworthy  method  of  discrimination  in  this  case. 

HEXAGONAL  MICA  SECTION. 

Biotite  crystallises  in  the  hexagonal  (rhombohedral) 
system.2  Colour,  black  or  dark  green.  Very  thin  laminae, 
appear  brown,  greenish,  or  red  by  transmitted  light  Che- 
mical composition,  silicate  of  magnesia,  potash,  iron,  and 

1  Memoire  sur  les  Roches  dites  Phitoniennes  de  la  Belgiqiie,  Poussin 
and  Renard.     Bruxelles,  1876,  p.  164. 

2  According  to  Descloizeaux  some  specimens  of  biotite  are  optically 
biaxial,  but  the  observed  divergence  of  the  optical  axes  is  veiy  slight. 
In  these  cases  the  mineral  must  be  regarded  as  rhombic  in  crystallisa- 
tion, and  closely  related  to,  if  not  identical  with,  phlogopite. 


136  The  Rudiments  of  Petrology. 

lalumina.  The  percentage  composition  of  the  mineral 
varies  considerably.  The  basal  cleavage  is  highly  perfect, 
and  the  laminae  are  flexible  and  elastic,  as  in  other  members 
of  the  mica  group.  The  mineral  is  only  slightly  acted  upon 
by  hydrochloric  acid,  but  is  decomposed  by  sulphuric  acid, 
^leaving  a  residue  of  glistening  scales  of  silica. 

Under  the  microscope,  crystals  which  lie  parallel  to  the 

basal  plane  appear  dark  between  crossed  Nicols,  but  sec- 

tions taken  in  other  directions  through  the  crystals  show 

_yery  strong  dichroism.     As  observed  by  A.  von  Lasaulx, 

hornblende,    tourmaline   and   epidote   are    the   only   other 

minerals  which  exhibit  equally  strong  dichroism.     Sections 

transverse  to  the  basal  plane  show  a  fine  striation,  which 

represents  the  cleavage,  and  the  crystals  often  appear  frayed 

\out  at  the  ends. 

Lepidomelane  occurs  in  small  six-sided  tabular  crystals, 
or  in  aggregations  of  minute  scales.  Colour,  black.  Lustre, 
adamantine  or  somewhat  vitreous.  Easily  decomposed  by 
hydrochloric  acid,  leaving  a  fine  scaly  residue  of  silica. 
In  chemical  composition  it  is  an  iron-potash  mica.  An 
analysis  of  lepidomelane  from  Carlow  Co.,  Ireland,  by 
Haughton,  gives 

SiO2  =  35-55.  A12O3=  17-08.  Fe2O3  =  237o.  FeO  =  3'55.  MnO  = 
1-95.     MgO  =  3'o/.     CaO  =  o-6i.      Na2O 


This  mica  occurs  in  some  of  the  Irish,  and,  according  to 
Allport,  in  some  of  the  Cornish  granites.  It  is  also  found 
in  certain  Swedish  rocks.  Lepidomelane  is  usually  regarded 
as  hexagonal,  and  consequently  as  optically  uniaxial,  but  some 
crystals  have  been  observed  which  appear  to  be  biaxial,  a 
very  slight  separation  of  the  axes  being  discernible. 

CHLORITE. 

Crystalline  system  hexagonal.  It  occurs  sometimes  in  a 
granular  form  ('  Peach  '),  sometimes  in  small  green  crystals 
and  scales.  Its  formula  is  2  (2  RO,  Si  O2)  +  A12O3,  sH2O 
in  which  RO  signifies  MgO  and  FeO.  The  average  per- 


Chlorite.     Talc.     Tourmaline.  137 

centage  of  magnesia  is  about  34  and  that  of  water  over  12. 
It  is  essentially  a  product  of  the  decomposition  of  other 
minerals.  When  heated  in  a  glass  tube  it  gives  off  water. 
Before  the  blowpipe  it  is  fusible  with  difficulty  on  thin 
edges.  It  is  decomposed  by  HC1  and  completely  soluble 
in  hot  H2  SO4.  Chlorite  is  optically  uniaxial,  consequently 
a  thin  crystal  of  chlorite,  when  lying  with  its  basal  plane  at 
right  angles  to  the  axis  of  vision,  exibits  no  dichroism,  but 
sections  of  chlorite  crystals  taken  either  obliquely,  or  at 
right  angles  to  the  basal  plane,  show  feeble  dichroism,  giving 
a  change  from  pale  to  deep  tints  of  green  when  examined 
with  a  single  Nicol,  although,  as  a  rule,  no  actual  difference 
of  colour  is  discernible.  Chlorite  often  contains  fluid  lacunae. 
It  frequently  forms  fibrous  and  radiate  aggregates.  Mag- 
netite and  radiating  nests  of  actinolite  are  commonly 
associated  with  chlorite.  Clinochlore,  which  is  monoclinic 
in  crystallisation  and  optically  biaxial,  also  frequently  occurs 
in  admixture  with  chlorite. 

TALC. 

Crystalline  system  rhombic  ?  or,  according  to  some  au- 
thors, hexagonal.  Has  a  highly  perfect  basal  cleavage. 
Cleaved  plates  are  flexible,  but  not  elastic  :  by  this  character 
it  may  be  distinguished  from  mica.  Hardness  =  i.  Crystals 
are  rare.  Colour  silvery- white  to  various  shades  of  green. 
Lustre  pearly.  Unctuous  to  the  touch.  Formula  (Mg  O)  6 
(Si  O2)  7.  Before  the  blowpipe  it  turns  white  and  exfoliates. 
It  is,  neither  before  nor  after  ignition,  soluble  in  either  hydro- 
chloric or  sulphuric  acids,  thus  differing  from  chlorite.  It 
also  differs  from  chlorite  in  showing  no  dichroism.  Under 
the  microscope  it  appears  as  imperfectly  developed  scales, 
with  a  fibrous  structure:  these  often  have  frayed  margins. 
Steatite  may  be  regarded  as  a  massive  variety  of  this  mineral. 

TOURMALINE. 

Crystalline  system  hexagonal  (rhombohedral).  The  crys- 
tals commonly  occur  in  long  prismatic  forms.  The  develop- 


138  The  Rudiments  of  Petrology. 

ment  is  often  hemihedral,  thus  giving  rise  to  triangular  prisms. 
The  terminations  of  tourmaline  crystals  are  frequently  com- 
posed of  a  great  number  of  faces,  and  the  one  termination 
differs  from  the  other.  The  crystals,  when  heated  and 
freely  suspended,  exhibit  polar  electricity,  a  phenomenon 
which  usually  accompanies  hemimorphism.  Cleavage 
rhombohedral  but  very  imperfect.  Crystals  generally 
striated  longitudinally.  They  often  form  radiate  groups. 
The  chemical  composition  of  tourmaline  is  very  variable. 
Tiae  general  formula  is  3R2O3 .  SiO2  +  aRO  .  SiO2.  All  the 
varieties  contain  silicate  of  alumina  with  about  4  to  10 
per  cent,  of  boracic  acid,  some  contain  protoxide,  some 
peroxide,  and  some  both  protoxide  and  peroxide  of  iron. 
Magnesia,  soda,  lime,  phosphoric  acid,  lithia,  and  fluorine 
also  occur  in  some  varieties.  The  fusibility  of  some  varieties 
is  effected  with  more  or  less  difficulty  before  the  blowpipe. 
Others  fuse  with  comparative  ease;  in  some  cases  the  fusion 
is  accompanied  by  intumescence.  Heated  with  powdered 
fluor-spar  and  bi-sulphate  of  potash,  all  the  varieties  impart 
to  the  oxidising  flame  the  green  colouration  indicative  of 
boracic  acid.  Tourmaline  is  strongly  doubly-refractive  and 
strongly  dichroic.  In  thin  section,  by  ordinary  transmitted 
illumination,  crystals,  or  portions  of  crystals,  frequently  ex- 
hibit a  deep  blue  colour  :  this  is  especially  the  case  when  the 
tourmaline  is  associated  with  quartz.  Transverse  sections  of 
prisms  often  afford  triangular  forms.  Although  in  its  strong 
dichroism  it  resembles  hornblende,  the  latter  mineral  may 
be  distinguished  from  it  by  its  well-marked  cleavage-planes. 
Biotite  is  not  likely  to  be  mistaken  for  tourmaline  owing  to 
the  lamellar  structure  of  the  former  mineral:  a  structure 
almost  always  visible  in  those  sections  of  biotite  which  are 
not  cut  parallel  to  the  basal  plane  and  which  exhibit 
dichroism.  Tourmaline  contains,  as  a  rule,  few  inclosures, 
the  most  common  being  fluid  lacunae.  The  black  variety 
of  tourmaline  is  termed  schorl,  and  is  of  frequent  occurrence 
in  certain  rocks,  especially  in  granites,  which  generally 


Epidote.  139 

become  schorlaceous  near  their  contact  with  other  rocks. 
In  such  cases  it  is  not  uncommon  to  find  the  schorl  segre- 
gated into  nests,  OT  small  spheroidal  masses. 

EPIDOTE. 

Crystalline  system  monoclinic.  Cleavage  parallel  to  the 
orthopinakoid  perfect,  parallel  to  the  basal  plane,  very 
perfect.  The  cleavage  planes  intersect  at  ai\  angle  of 
115°  24'.  Colour  usually  green  or  yellowish-green,  sometimes 
brown.  Epidote  occurs  at  times  in  a  granular  or  massive 
condition;  the  crystals  commonly  form  radiate  or  fan-shaped 
groups,  and  either  form  nests,  or  line  fissures  and  cavities. 
The  mineral  is  essentially  an  alteration-product,  consisting 
of  silicate  of  alumina,  lime,  and  peroxide  of  iron,  with 
variable  amounts  of  oxide  of  manganese  and  water.  Before 
the  blowpipe  it  usually  gives  an  iron  or  manganese  reaction 
with  fluxes.  There  are  several  varieties,  one  containing 
about  14  per  cent,  of  manganese  oxides. 

Under  the  microscope  epidote  exhibits  strong  pleo- 
chroism.  When  tested  with  a  single  Nicol  it  does  not,  how- 
ever, show  this  property  so  strongly  as  hornblende. 

Inclosures  are  rare  in  epidote;  nevertheless  fluid  lacunae 
have  been  observed  in  it.  The  way  in  which  epidote  occurs 
in  rocks  forming  little  fan-shaped  aggregates  of  radiating 
needles  or  fibres  along  fissures,  &c.,  its  bright  yellowish- 
green  colour,  and  its  strong  pleochroism — all  of  these  charac- 
ters taken  in  conjunction  serve,  as  a  rule,  to  distinguish  it 
from  other  minerals.  Epidote  often  forms  little  fringes 
around  hornblende  crystals.  Chlorite  is  one  of  the  minerals 
which  most  resembles  it  in  its  mode  of  occurrence.  In 
doubtful  cases,  the  respective  strength  of  dichroism,  the 
absence  of  dichroism  in  plates  of  chlorite  viewed  perpen- 
dicularly to  the  basal  plane,  the  hexagonal  form  of  such 
chlorite  plates,  and  the  difference  in  the  colour  of  the  two 
minerals,  are  points  which  should  be  looked  for  and  taken 
into  consideration. 


1 40  The  R  ndimen  ts  of  Petrology. 

SPHENE  (Titanite) 

crystallises  in  the  monoclinic  system  ;  common  form,  the 
oblique-rhombic  prism.  The  crystals  are  usually  thin  and 
have  sharp  wedge-like  edges.  Colours,  brownish-grey,  yellow, 
;  green,  and  black.  Transparent  to  opaque. 

Approximate    chemical    composition.     Silica=3o  — 35, 
titanic  acid= 33 — 43,  lime  =  21  —  33  Per  cent.     The  formula 
jnay  be  written  :— CaO,  2SiO2  +  CaO,  2TiO2. 

Under  the   microscope,   sections   of  sphene   generally 
appear,  by  transmitted  light,  brownish  yellow,  yellow,  some- 
»jimes  red,  reddish  brown,  and  colourless  ;  'they  show  distinct 
though  not  strong,  pleochroism,  except  when  the  sections 
are    very    clear    and   colourless   or   of    extreme   thinness, 
j  Infiltration  products,  consisting  either  of  hydrous  or  anhy- 
i  drous  oxide  of  iron,  are  sometimes  seen  between  the  planes 
L-of  cleavage  in  sphene.     Intergrowths  of  sphene  and  horn- 
blende have  been  observed  by  Groth,  in  the  syenite  of  the 
Plauenschen  Grund,  near  Dresden.  /  Frequently  the  crystals 
of  sphene  appear  cloudy  or  imperfectly  translucent.     The 
sections  usually  present  very  characteristic   wedge-shaped 
forms.     As  a  rule  sections  of  sphene  appear  to  be  remarkably 
free  from  inclosures  of  other  minerals. 

GARNET  GROUP. 

All  the  members  of  this  group  crystallise  in  the  cubic 
system,  the  common  forms  being  either  the  rhombic  dode- 
cahedron or  the  icositetrahedron.  The  cleavage  is  parallel 
to  the  faces  of  the  dodecahedron.  The  garnets  vary  in 
hardness  from  6'5  to  7*5.  They  have  a  subconchoidal  or 
uneven  fracture,  and  they  all  afford  an  approximately  white 
streak.  Before  the  blowpipe  most  of  them  fuse  easily,  but, 
in  accordance  with  the  different  chemical  composition  of  the 
various  species,  they  give  different  blowpipe-reactions  with 
the  fluxes,  in  which  the  iron  reactions  dominate. 

In  chemical  composition  the  garnets  are  essentially 
unisilicates  of  different  sesquioxides  and  protoxides.  The 


Garnet  Group.  ^141 

'*/,         \/t- 

sesquioxides  are  those  of  aluminium,  iron,  and  chromium ;  / 

sometimes  also  of  manganese,  while  the  protoxides  ar-0'those 
of  iron,  calcium,  magnesium,  or  manganese.     *-/jr  > 

The  principal  sub-species  have  the  following  6qmposi-  J 
tion  :— 

1.  Lime-alumina  garnet,  6CaO,  3SiO2  +  2A12O3,  3SiO2.        «  f 

2.  Magnesia-alumina  garnet,  6MgO,  3SiO2  +  2A12O3,  3SiO.,. 

3.  Iron-alumina  garnet,  6FeO,  3SiO2  +  2A12O3,  3SiO2. 

4.  Manganese-alumina  garnet, 6MnO,  3SiO2  +  2A12O3,  3SiO2. 

5.  Iron-lime  garnet,  6CaO,  3SiO2  + 2Fe2O3,  3SiO2. 

6.  Lime-chrome  garnet,  6CaO,  3SiO2 -f  2Cr2O3,  3SiO2. 

The  percentage  of  silica  in  the  various  sub-species  is 
tolerably  uniform,  ranging  from  35  to  40  per  cent. 

For  descriptions  of  the  general  characters  of  the  different 
sub-species  of  the  garnet  group,  the  student  may  refer  to 
any  manual  of  mineralogy,  since  only  the  microscopic  cha- 
racters of  the  group  will  be  described  here.  . 

In  thin  sections  of  rocks,  garnets  frequently  appear  under 
the  microscope  merely  as  rounded  granules,  somewhat 
resembling  spots  of  gum,  generally  colourless  or  of  clear 
reddish  tints,  but  sometimes,  as  in  the  case  of  picotite,  of  a 
deep  brownish  or  reddish-brown  colour,  and  forming  irre- 
gular gummy-looking  streaks.  When  definitely  formed 
crystals  occur,  they  afford,  as  a  rule,  four-sided,  six-sided, 
and  eight- sided  sections.  They  are  mostly  traversed  by 
irregular  cracks.  The  crystals  occasionally  appear  to  have 
formed  around  a  nucleus  of  some  other  mineral,  such  as 
quartz,  epidote,  &c.  The  minerals  usually  inclosed  in 
garnets  are  magnetite,  hornblende,  tourmaline,  quartz,  occa- 
sionally apatite  and  augite,  and  colourless  microliths,  forming 
narrow  prisms,  whose  nature  has  not  yet  been  determined. 
Garnets  also  contain  at  times  cavities  in  the  form  of  the 
rhombic  dodecahedron  (negative  crystals).  Although  garnets 
occur  in  some  eruptive  rocks,  yet  they  are  most  plentiful  in 
those  which  have  undergone  strong  metamorphism,  and 


142  The  Rtidiinents  of  Petrology. 

they  are  especially  common  in  granulite,  gneiss,  talcose,  and 
chloride  slates,  and  other  schistose  metamorphic  rocks. 
They  also  occur  in  serpentines,  and  granular  and  crystalline 
limestones.  The  Belgian  hone-stones  (coticules]  consist  in 
great  part,  according  to  Prof.  Re'nard,  of  the  manganese 
garnet  spessartine.1  Since  they  crystallise  in  the  cubic 
system  they  exhibit  single  refraction.  Some  garnets  have, 
however,  been  observed  to  possess  double  refraction,  but 
these  anomalous  examples  have  not  yet  been  fully  investi- 
gated. 

Idocrase,  or  Vesuvian,  is  in  its  chemical  composition 
closely  allied  to  the  lime-alumina  garnets,  but  crystallises  in 
the  tetragonal  system.  According  to  Sorby  it  often  contains 
fluid  lacunae  in  which  very  numerous,  but  undetermined 
crystals  occur. 

TOPAZ 

is  not  a  mineral  of  common  occurrence  in  rocks.  It  only 
attains  importance  as  a  rock-component  in  the  '  Topazfels ' 
of  Schneckenstein  in  Saxony  ;  still  it  is  occasionally  met 
with  as  an  accessory  constituent  of  certain  rocks. 

Its  crystallisation  is  rhombic,  but  it  also  occurs  in  a 
granular  condition.  It  is  infusible  before  the  blowpipe  and 
insoluble  in  acids.  Under  the  microscope  it  is  strongly 
doubly-refracting  ;  but,  although  it  is  dichroic,  the  dichroism 
is  so  weak  that  in  thin  sections  it  is  scarcely  perceptible. 
Fluid  lacunae  are  common  in  crystals  of  topaz;  the  liquid  is 
sometimes  a  saline  solution,  sometimes  liquid  carbonic 
acid.  The  fluid  inclosures  in  topaz  were  first  investigated 
by  Brewster  in  1845,  an(^  nave  subsequently  been  examined 
by  Sorby  and  Vogelsang.  Rosenbusch  notices  the  inclu- 
sion in  topaz  of  scales  of  hematite,  and  of  black  flecks  and 
granules  which  show  no  metallic  lustre,  and  suggest  the  idea 

1  *  Memoire  sur  la  Structure  et  la  composition  Mineralogique  du 
Coticule,'  A.  Renard.  Bruxelles,  1877,  tome  xli.  des  Mhnoires couronnes 
de  r Academic  royale  de  Belgique. 


Zircon.     Andalusite.  143 

of  carbonaceous  matter;  but,  as  they  undergo  no  change  when 
heated,  the  notion  is  regarded  as  erroneous.1 

ZIRCON. 

This  mineral  is  met  with  in  some  lavas  and  volcanic 
ejected  blocks,  also  in  zircon- syenite.  It  crystallises  in  the 
tetragonal  system.  Sections  of  some  varieties  of  zircon 
display  strong  dichroism,  while  in  others  it  is  scarcely 
perceptible. 

In  chemical   composition  it  is  essentially  silicate  of  zir-, 
conia  and   since  it  contains  no  protoxides  it  is  but  little 
liable  to  undergo  alteration.     It,  however,  at  times  becomes  j 
hydrous,  loses  silica,  and  becomes  partly  replaced  by  other  I 
substances. 

ANDALUSITE. 

Crystallographic  system,  rhombic.  The  common  form 
is  a  combination  of  the  rhombic  prism  oo  P,  the  macro- 
domes  P  oo  and  the  basal  planes  oP.  The  crystals,  especially 
the  larger  ones,  are  usually  incrusted  with  and  penetrated  by 
scales  of  mica.  At  times  they  are  feebly  translucent  and 
very  rarely  transparent.  The  cleavage  is  very  indistinct. 
Andalusite  sometimes  occurs  in  a  granular  condition. 

Its  chemical  composition  is  represented  by  the  formula 
A12O3,  SiO2. 

Under  the  microscope  thin  sections  polarise  strongly  and 
show  well-marked  pleochroism.  Alteration  is  denoted  by 
the  development  of  a  fibrous  structure  which  usually  runs  in 
definite  directions.  In  some  crystals  of  andalusite  carbo- 
naceous matter  occurs  ;  but,  as  a  rule,  the  mineral  is  very 
free  from  inclosures  of  foreign  substances.  It  is  most  com- 
monly met  with  in  mica  schist  and  slaty  rocks. 

The  variety  chiastolite  or  made,  so  named  from  the  spots 
and  cruciform  markings  which  occur  in  the  interior  of  the 
crystals,  is  met  with  only  in  slates  which  have  undergone 

1  Rosenbusch,  Mik.  Physiog.  d.  Min.  p.  282.     Stuttgart,  1873. 


144  The  Rudiments  of  Petrology. 

alteration  from  proximity  to  eruptive  rocks,  as  in  the 
Skiddaw  slate  where  it  nears  the  granite.  The  changes  which 
take  place  in  this  district,  as  described  by  Mr.  J.  Clifton 
Ward,1  consist  first  in  the  faint  development  of  oblong  or 
oval  spots  in  the  slate,  together  with  a  few  crystals  of  chias- 
tolite ;  the  latter  then  become  quite  numerous  and  well 
developed,  constituting  the  true  chiastolite  slate.  This 
passes  into  a  harder,  foliated,  and  spotted  rock  which  Mr. 
Ward  regards  as  knotenschiefer,  the  spots  being  imperfectly 
developed  chiastolite  crystals,  accompanied  by  more  or  less 
mica  and  quartz,  while,  in  the  immediate  neighbourhood  of 
the  granite,  the  rock  passes  into  mica-schist.  Crystals  of 
chiastolite  afford  sections  which  vary  considerably  in  form, 
some  giving  rhomboidal  outlines  with  a  dark  nucleus  in 
the  centre  of  the  section.  The  boundaries  of  the  crystal 
section  are  usually  sharply  defined,  but  the  nucleus  which, 
under  the  microscope,  may  generally  be  resolved  into  a 
mass  of  dark  flakes  and  granules,  is  not  very  distinctly 
separable  from  the  more  or  less  translucent  surrounding 
matter  of  the  crystal,  appearing  to  have  a  hazy  boundary. 
Zirkel  states  that  in  such  nuclei  a  linear  arrangement  of 
granules  or  flakes  may  sometimes  be  discerned  passing 
from  the  centre  to  the  angles  of  the  section  (i.e.  to  the 
lateral  edges  of  the  rhombic  prism),  but  he  adds  that  this 
arrangement  is  seldom  so  clearly  perceptible  in  microscopic 
individuals  as  in  the  larger  crystals  in  which  such  divisional 
markings  are  visible  to  the  naked  eye. 

KYANITE. 

Crystalline  system  triclinic.  Chemical  composition 
similar  to  that  of  andalusite.  The  crystals  are  generally 
long  prisms,  which  appear  broad  in  one  direction  and  narrow 

1  Memoirs  of  the  Geological  Survey  of  England  and  Wales  (The 
Geology  of  the  northern  part  of  the  English  Lake  District),  pp.  9-12. 
See  also  Abhand.  zur  Geol.  Specialkarte  v.  Elsass-Lothringen.  Die 
Steiger  Schiefer  u.  ihre  contactzone  an  den  Granititen  v.  Barr-Andlau 
u.  Hochwald.  H.  Rosenbusch,  pp.  210-215.  Strassburg,  1877. 


Apatite.  145 

in  the  other.  The  cleavages  are  prismatic  and  basal,  the 
former  being  tolerably  distinct  parallel  to  the  broad  faces, 
but  less  so  in  the  direction  of  the  narrow  ones.  The  basal 
cleavage  enables  the  prisms  to  be  easily  broken  in  a  trans- 
verse direction.  The  crystals  are  seldom  terminated.  It  also 
occurs  in  radiating  or  interlacing  fibrous  conditions.  It  is 
transparent,  with  a  vitreous  lustre,  and  is  coloured  blue,  or 
greyish-blue  and  white,  individual  prisms  often  showing  a 
succession  of  blue  and  colourless  bands  which  graduate 
into  one  another.  The  mineral  is  pleochroic,  but  this  is  not 
perceptible  in  thin  sections.  Inclosures  of  other  minerals 
are  rare  in  kyanite. 

APATITE. 

Crystalline  system  hexagonal.  The  crystals  are  usually  1 
combinations  of  the  hexagonal  prism  and  basis,  sometimes 
modified  by  faces  of  the  hexagonal  pyramid.  Although 
crystals  of  apatite  several  inches  in  length,  and  sometimes 
of  much  greater  size,  are  occasionally  found,  still  the  majority 
of  those  which  enter  into  the  composition  of  rocks  are  of 
microscopic  dimensions.  These  little  prisms  are  usually 
very  long  in  proportion  to  their  breadth. 

The  hardness  of  apatite  is  5.  /It  contains  from  90  to 
per  cent,  phosphate  of  lime,  arid  its  formula  is  either 

3(Ca3P208)  +  CaCl2  or  3  (Ca3P2O8)  +  CaF2, 

according  to  whether  the  mineral  contains  chloride,  or 
fluoride  of  calcium.  It  frequently  contains  both. 

The  detection  of  phosphoric  acid  in  rocks  is  best  effected 
by  finely  pulverising  a  tolerably  large  sample,  digesting  it 
in  hydrochloric  acid,  filtering  off  the  solution,  and  treating 
it  with  molybdate  of  ammonium.  If  phosphoric  acid  be 
present,  a  yellow  precipitate  will  be  formed,  and  the  pre- 
cipitation, which  usually  takes  place  very  slowly,  may  be 
accelerated  by  frequent  stirring  with  a  glass  rod.  /  Most] 
of  the  phosphoric  acid  which  exists  in  rocks  probably) 


146  The  Rudiments  of  Petrology. 

(occurs  in  the  form  of  apatite ;  except  in  cases  where, 
instead  of  being  minutely  disseminated,  it  occurs  as  phos- 
phorite. 

Under  the  microscope   apatite   appears    in    elongated 

hexagonal  prisms,  which,  when   cut   longitudinally,  afford 

.rectangular  sections,  and  transversely,  hexagonal  ones. /The 

i  boundaries  of  the  crystals  are  always  sharply  denned.     Since 

the  mineral  is  uniaxial,  the  sections  taken  at  right  angles  to 

^the  principal  axis  appear  dark  between  crossed  Nicolsr^In 

|  the  colourless  apatite  crystals,  which  usually  occur  in  rocks, 

j  no  dichroism  can  be  discerned,  although  in  some  coloured 

/'  varieties  of  the  mineral  it  is  quite  perceptible.  /Apatite  crys- 

^tals  often  contain  light  greyish  or  yellowish  dusty  matter, 

the  nature  of  which  has  not  yet  been  determined,  although, 

from  an  examination  of  large  crystals  containing  somewhat 

similar  impurities,   it  has   been  inferred  that  the  dust  may 

consist  partly  of  magnetite  granules,  and  partly  of  acicular 

microliths,  together  with  inclosures  of  glass  and  of  fluid, 

the    former   showing  motionless,  and  the   latter   movable, 

bubbles. 

In  examining  some  large  pellucid  apatite  crystals  from 
the  Val  Mayia,  Fritzgartner  found  them  to  contain  small 
elongated  hexagonal  prisms  and  pores  filled  with  liquid. 
The  latter  varied  in  form  and  size,  but  were  mostly  round. 
The  hexagonal  prisms  lay  with  their  longer  axes  parallel  to 
the  basal  plane  of  the  containing  crystal,  and  appear  to 
follow  irregular  curves,  and  to  be  arranged  in  no  directions 
corresponding  with  the  other  axes  of  the  crystal  which  con- 
tained them.1 

Apatite  crystals  sometimes  envelope  a  black'  opaque 
substance  which  corresponds  in  its  boundaries  with  the  boun- 
daries of  the  containing  crystal,  the  latter  often  forming 
little  more  than  a  clear,  narrow  margin  around  this  dark 
nucleus.  Zirkel  notes  the  occasional  symmetrical  disposition 
of  six  small  apatite  crystals  around  a  larger  one. 

1  Private  communication  from  Dr.  R.  Fritzgartner. 


Apatite.     Rutile.  147 

Minute  crystals  of  apatite  may  be  distinguished  from 
those  of  felspars  by  their  hexagonal  transverse  sections. 
They  may  usually  be  distinguished  from  nepheline  by  oc- 
curring on  a  much  smaller  scale,  and  being  of  much  greater 
length  in  proportion  to  their  breadth,  so  that  they  afford 
rectangular  sections  which  are  generally  much  longer,  and 
hexagonal  sections  which  are  much  smaller,  than  the  cor- 
responding ones  derived  from  nepheline.  The  student 
should  also  be  on  his  guard  against  mistaking  small  apatite 
needles  for  colourless  microliths  of  hornblende,  augite,  &c. 
Apatite  crystals  seem  to  be  rather  gregarious,  often  colo- 
nising in  certain  portions  of  a  rock,  and  being  nearly  absent 
in  others. ,/ It  is  of  all  minerals  one  of  the  most  widely? 
distributed,  occurring  in  a  vast  number  of  rocks  of  very| 
diverse  mineral  composition,  often  being  present  only  inj 
very  minute  quantity/  It  is  even  regarded  by  Zirkel  as  of 
more  common  occurrence  than  magnetite. 

Apatite  crystals  usually  remain  clear  and  fresh  long  afterj 
the  other  mineral  constituents  of  a  rock  have  decomposed.} 
Although    so    comparatively   invulnerable    to    the    natural 
agents  which  decompose  rocks,/it  is  soluble  in  hydro-chloric} 
acid. 

Asparagus-stone  and  moroxite  are  names  given  to  yellow- 
ish green  and  blueish  green  varieties  of  apatite.  In  Canada 
the  latter  mineral  occurs  in  a  bed  ten  feet  thick  passing 
from  North  Elmsley  into  South  Burgess.  Three  feet  of  this 
bed  consist  of  pure  sea-green  apatite,  while  the  remainder 
is  made  up  of  apatite  and  limestone,  in  which  crystals  of 
pyroxene  and  phlogopite  also  occur.1 

RUTILE 

(TiO2),  which    crystallises    in   the   tetragonal  system,   ap- 
pears deep  red  or  brown  when  seen  in  thin  section  under 
the   microscope.     It  is  not  very  strongly   dichroic.      The 
crystals  are  often  seen  t©  be  traversed  by  thin   plates  or 
1  System  of  Mineralogy',  Dana,  5th  edition,  p.  533. 
L2 


148  The  Rudiments  of  Petrology. 

strise,  and  by  included  crystals  which  follow  the  direction 
of  the  principal  axis,  and  that  of  the  twinning  plane  co  P. 
A  good  figure,  showing  this  structure  along  the  plane  of 
geniculation,  is  given  in  Rosenbusch's  *  Mikroscopische 
Physiographic,5  vol.  i.  p.  187,  to  which  work  the  student  is 
referred  for  further  particulars  respecting  the  microscopic 
character  of  this  mineral. 

CASSITERITE, 

(SnO2)  crystallises  in  the  tetragonal  system.  The  crystals, 
when  examined  in  thin  section  under  the  microscope,  appear 
by  transmitted  light  of  a  honey-yellow  colour. 

CALCSPAR  (Calcite). 

Crystalline  system  rhombohedral.  The  crystals  vary 
greatly  in  the  combinations  which  they  present.  The  most 
common  forms  are  rhombohedra,  scalenohedra,  and  hexa- 
gonal prisms,  terminated  by  basal  planes  or  by  planes  of  a 
rhombohedron.  The  simple  rhombohedral  forms  with 
angles  of  105°  5'  and  74°  55'  are  those  which  chiefly  occur 
in  rocks,  and  the  cleavage  corresponding  with  this  form  is 
commonly  to  be  recognised  in  granular  aggregates  of 
carbonate  of  lime.  Chemical  formula  CaCO3. 

The  mineral  frequently  contains  some  magnesium  or 
iron  replacing  part  of  the  calcium.  In  some  cases  it.  is  im- 
pregnated with  sand,  as  in  the  well-known  crystals  from 
Fontainebleau,  which  sometimes  contain  over  60  per  cent,  of 
that  material.  When  treated  with  acids,  calcspar  effervesces, 
giving  off  carbonic  anhydride.  It  is  easily  scratched  with  a 
knife,  since  it  only  has  a  hardness  of  3.  Before  the  blowpipe 
it  is  infusible,  becoming  strongly  luminous  ;  and,  giving 
off  its  carbonic  anhydride,  it  is  reduced  to  quick-lime,  the 
crystal,  if  previously  transparent,  becoming  opaque,  white, 
and  pulverulent. 

Under  the  microscope,  sections  of  calcspar  show  very 
strong  double  refraction,  which  may  be  observed  by  using 


Calcspar.     Quartz.  149 

the  analyser  alone.  The  planes  of  cleavage  which  intersect 
one  another  are  also  generally  visible,  while  by  polarised 
light  it  is  common  to  find  that  the  separate  granules  which 
constitute  crystalline  aggregates  are  composed  of  numerous 
lamellae  which  polarise  in  different  colours,  and  which 
denote  a  system  of  twinning  parallel  to  the  face— \  R. 
The  lines  of  demarcation  between  the  lamellae  are  sharply 
denned,  and  run  parallel  to  one  another  in  the  same 
individual  or  granule  ;  but  the  planes  of  twinning  in  any  one 
granule  observe  no  relation  to  those  belonging  to  adjacent 
granules.  This  twin  structure  may  be  well  seen  in  crystalline 
limestones,  statuary  marble,  &c.  It  is  very  characteristic 
of  calcspar,  and  serves  as  a  rule  to  distinguish  the  mineral 
from  dolomite,  which  seldom  shows  any  such  structure. 
Reusch  has  demonstrated  that  a  similar  twin  structure  may 
be  artificially  produced  in  calcspar  by  pressure.  Inclosures 
of  other  minerals  are  common  in  calcspar.  Iron  pyrites, 
native  copper,  copper  pyrites,  copper  glance,  and  a  large 
number  of  other  minerals,  are  at  times  met  with  in  calcspar, 
but  most  of  these  inclosures  are  visible  without  the  aid  of 
a  microscope.  The  fluid  inclosures  which  sometimes  occur 
in  calcite  are  generally  regarded  either  as  water,  or  as 
water  containing  carbonic  acid  or  bicarbonate  of  lime. 

QUARTZ.     )\ 

Crystalline  system  hexagonal ;  or,  as  indicated  by  the 
occasionally  occurring  tetartohedral  faces,  rhornbohedral. 
The  usual  forms  are  either  hexagonal  pyramids,  or  combi- 
nations of  the  hexagonal  pyramid  and  hexagonal  prism. 
In  the  former  case  the  sections  parallel  to  the  principal 
axis  yield  rhomboidal  figures,  in  the  latter  elongated  hexa- 
gons, while  in  both  instances  the  sections  transverse  to 
the  principal  axis  are  regular  hexagons.  Twinning  is 
common,  sometimes  giving  rise  to  geniculation,  sometimes  ' 
producing  cruciform  arrangements,  at  others  causing  irre- 
gular interpenetration  of  dissimilar  parts  of  the  crystal,  the 


150  The  Rudiments  of  Petrology. 

positive  rhombohedral  faces  being  irregularly  penetrated  by 
the  negative,  and  vice  versa.  The  chemical  composition 
of  quartz  is  Si  O2,  with  occasional  impurities,  such  as  iron 
oxides,  titanic  acid,  &c.  Quartz  is  infusible  before  the 
blowpipe,  insoluble  in  all  acids  except  fluoric  acid.  It  is 
also  more  or  less  acted  upon  by  a  hot  solution  of  potash ;  in 
the  case  of  the  purer  crystallised  varieties,  but  very  slightly. 
In  the  compact  and  crypto- crystalline  conditions  its  solu- 
bility in  this  reagent  is,  however,  according  to  Rammelsberg 
somewhat  greater.1  The  hardness  of  quartz  is  7,  the  point 
of  a  penknife  producing  no  effect  upon  it,  unless  it  be  in  a 
finely  granular  condition,  when  the  point  may  simply  rake 
up  and  detach  a  few  granules,  upon  which,  however,  it  is 
unable  to  make  any  impression.  In  this  instance,  as  in 
others,  it  behoves  the  student  to  be  on  his  guard  in  testing 
the  hardness  of  granular  or  finely  crystalline  substances, 
to  distinguish  between  the  disintegration  of  granular  struc- 
tures and  true  streak.  The  pressure,  however,  required  to 
scrape  off -the  smallest  trace  of  dust  from  a  quartzite,  or  from 
granular  conditions  of  quartz,  is  very  considerable.  The 
fracture  of  quartz  is  conchoidal.  The  specific  gravity  =  2*65. 

Sections  of  quartz  appear  clear  and  pellucid  under  the 
microscope.  They  show  circular  polarisation,  and  exhibit, 
in  some  sections,  magnificent  variegations  of  colour.  The 
plane  of  polarisation  is  sometimes  right-handed,  sometimes 
left-handed,  in  its  rotation,  and  both  the  right-handed  and 
left-handed  phenomena  of  rotation  are  at  times  seen  in  the 
same  crystal.  For  further  information  upon  this  subject  the 
student  is  referred  to  the  works  of  Brewster,  Descloizeaux, 
Groth,  and  others. 

Quartz  is  seen  frequently  to  contain  inclosures  of  other 
substances,  sometimes  as  crystals,  sometimes  in  the  form  of 
lacunae  filled  with  liquids,  &c.  These  inclosures  are  often 
visible  to  the  naked  eye,  but  the  microscope  commonly 
reveals  their  presence  in  vast  numbers.  The  crystals  of 
*  Ann.  cxii.  177. 


Quartz.  151 

most  frequent  occurrence  are  those  of  rutile  and  chlorite ; 
crystals  of  kyanite  are  also  occasionally  met  with.  Lacunae 
of  glass,  others  filled  with  gas,  and  others  containing  por- 
tions of  the  rock  matrix  in  which  the  crystals  are  imbedded, 
are  by  no  means  uncommon  in  some  rocks.  The  most 
numerous  lacunae,  however,  are  those  containing  liquids. 
The  liquids  are  frequently  pure  water;  sometimes  water  hold- 
ing carbonic  acid  in  solution,  sometimes  liquid  carbonic  acid, 
sometimes  a  supersaturated  solution  of  chloride  of  sodium, 
minute  crystals  of  rock  salt  being  visible  within  the  lacunae 
under  tolerably  high  powers.  These  lacunae  are  at  times 
completely  filled  with  the  fluid,  at  others  they  are  seen  to 
contain  bubbles  which  vary  in  magnitude  and  are  re- 
garded as  representing  the  diminution  of  volume  which 
the  fluid  in  the  cavity  has  undergone  during  the  cooling 
of  the  rock  mass  in  which  it  occurs.  Deductions  based 
upon  the  relative  volumes  of  the  fluids,  and  of  the  va- 
cuities in  such  cavities,  may  be  found  in  the  paper  com- 
municated to  the  Geological  Society  by  Mr.  Sorby  in  1858. 
It  is,  however,  deserving  of  note,  as  pointed  out  by  Mr. 
John  Arthur  Phillips,  that,  in  the  same  crystal,  cavities 
may  be  found,  some  completely  filled,  while  others  contain 
vacuities  whose  relative  volumes  to  those  of  their  sur- 
rounding fluids  vary  very  considerably.  The  bubbles  in 
these  lacunae  are  often  moveable,  being  displaced  either  by 
simply  turning  the  crystal,  or  more  usually  by  heating  it, 
in  which  case  the  bubble  undergoes  diminution  of  volume, 
or  even  disappears.  In  some  small  lacunae  diminutive  bub- 
bles, which  have  a  spontaneous  motion,  are  visible  under 
high  magnifying  powers.  These  bubbles  are,  however,  so 
minute  that  they  appear  frequently  as  mere  specks,  and  it 
needs  careful  and  steady  watching  to  see  their  motion,  which 
looks  like  a  tremulous  gyration,  often  resulting  in  a  com- 
paratively well-marked  change  of  place,  followed  perhaps  by 
a  pause,  to  be  again  succeeded  by  the  oscillations  and 
gyrations  already  mentioned. 


152  The  Rudiments  of  Petrology. 

Tridymite  is  a  form  of  silica  discovered  in  1866  by  Vom 
Rath.  It  occurs  in  very  small  six-sided  tabular  crystals.  The 
system  to  which  these  crystals  belong  has  not  yet  been  satis- 
factorily determined.  The  specific  gravity  is  2-2  to  2-3,  the 
same  as  that  of  opal.  The  crystals  occur  in  compound  groups, 
mostly  composed  of  three  individuals,  whence  the  name.  It 
is  found  in  the  sanidine  oligoclase  trachyte  of  the  Drachen- 
fels,  in  a  volcanic  porphyry  from  near  Pachucha  in  Mexico, 
in  a  porphyry  from  Waldbockelheim,  in  some  Hungarian 
liparites,  in  the  hornblende-andesites  of  Dubnik,1  in  a 
trachytic-looking  phonolite  from  Aussig,2  in  the  wolf-rock 
(phonolite)  described  by  Allport,  in  several  phonolites  de- 
scribed by  Mohl.3  It  has  also  been  mentioned  as  occurring 
in  some  Irish  rock,  but  the  author  is  unable  either  to  recall 
the  precise  locality,  or  to  find  the  reference.  According  to 
Vom  Rath,  the  discoverer,  it  is  doubly  refracting  and  opti- 
cally uniaxial. 

A  globular  condition  of  silica  has  been  lately  described 
by  Michel  Levy 4  as  occurring  in  the  euritic  porphyries  of 
Les  Settons,  and  similar  globular  conditions  of  silica  have 
also  been  observed  and  noticed  by  M.  Velain  in  a  quartz- 
trachyte  from  Aden.  The  former  author  regards  this  con- 
dition as  intermediate  between  the  crystallised  and  the 
colloid  forms  of  silica. 

The  following  extract  from  M.  Michel  Levy's  paper  will 
convey  an  idea  of  the  microscopic  characters  of  these  glo- 
bules :  '  Between  crossed  Nicols  one  is  surprised  to  see  such 
regular  globules,  the  centre  of  each  appearing  to  be  a  pole 
of  symmetry  with  four  extinctions  situated  at  right  angles  for 
every  total  revolution  of  the  section ;  one  is  therefore  forced 
to  conclude  that  they  are  composed  of  a  crystallised  sub- 
stance, and  are,  moreover,  orientated  in  an  unique  manner, 

1  H.  Rosenbusch,  Mikroskop.  Physiogr.  d.  massigen  Gesteine.    Stutt- 
gart, 1877,  p.  301. 

2  Ibid.  p.  225. 

3  H.  Mohl,  Basalte  u.  Phonolithe  Sachsens.    Dresden,  1873. 
'  Bull.  Soc.  Geol.  de  France,  3*  serie,  t.  v.  1877,  no.  3. 


Magnetite.  153 

Sometimes  the  extinction  is  simultaneous  over  an  entire 
globule,  sometimes  it  is  different  for  two  or  more  segments ; 
but  the  most  curious  peculiarity,  exhibited  by  the  concentric- 
ally-zoned globules,  lies  in  the  fact  that  two  adjacent  zones 
do  not  always  undergo  extinction  simultaneously.  Such  a 
globule  will  undergo  extinction  in  its  central  portion  and 
will,  at  the  same  time,  present  a  perfectly  regular  narrow 
border  which  is  still  illuminated ;  if  then  the  observer  con- 
tinue to  turn  the  section,  this  border  will  become  dark  while 
the  spherical  central  nucleus  will  in  its  turn  become  clear 
in  a  homogeneous  manner.' 

MAGNETITE. 

Crystalline  system  cubic.     It  usually  occurs  in  the  form  of 
the  octahedron,  sometimes  in  that  of  the  rhombic-dodeca- 
hedron, also  granular  and  massive.     Cleavage  parallel  to 
the  faces  of  the  octahedron.     Colour  black.     Streak  black. 
Strongly  magnetic  and  often  displays  polarity./  The  chemical 
formula  of  magnetite  is  FeO,  Fe2O3,  or  Fe3O4.  The  approx- 
imate percentage  composition  is  Fe2O3  =  69.     FeO  =  31. 
Magnetite  is  frequently  titaniferous.  /  It  [is  very  difficultly! 
fusible  before  the  blowpipe.     When  pulverised  it  is  com-  I 
pletely  soluble  in  hydrochloric  acid.     Even  in  the  thinnest  ' 
sections  magnetite  appears  opaque  under  the  microscope ;..x 
nevertheless,  when  it  has  undergone  alteration  either  into 
hematite  or  limonite  it  appears  feebly  translucent  at  times 
and   of  a  reddish  or   brownish   colour.     The   sections   of 
magnetite  crystals,  which  are  of  most  common  occurrence 
in  rocks,   present  square   forms   which   represent   sections 
passing   through  opposite  solid  angles   of  the  octahedron, 
or  triangular  forms  which  result  from  sections  taken  parallel 
to  one  of   the  faces   of  the   octahedron  (fig.  69).  /  Twin  I 
crystals  sometimes    occur,   the    twinning   taking 
place  on  a  plane  parallel  to  a  face  of  the  octahe-  _     ^ 
dron.    The  superposition  of  one  crystal  on  another 
sometimes    gives    rise    to    cruciform   figures.      Magnetite 


1 54  The  Rudiments  of  Petrology. 

very  usually  occurs  in  a  granular  condition  in  rocks,  'some- 
times in  coarse  irregular  grains,  sometimes  as  a  fine  dust, 
while  at  others  these  granules  form  segregations  which  give 
.  rise  to  rod-like  forms.  ''Occasionally,  as  in  some  basalts, 
magnetite  crystals  are  grouped  in  a  very  regular  manner, 
following  lines  which  are  frequently  disposed  at  right  angles. 
Such  groupings  are  not  merely  to  be  met  with  in  volcanic 
rocks,  but  also  in  furnace  slags,  and  their  arrangement  often 
seems  to  imply  the  rudimentary  stages  of  aggregation  which 
might  eventually  result  in  the  formation  of  a  large  crystal  from 
the  contiguous  development  of  smaller  ones. 

TlTANIFEROUS    IRON. 

Crystallisation  rhombohedral.  It  mostly  occurs  in  tabular 
forms  with  the  basal  planes  largely  developed  and  with 
hexagonal  boundaries.  Titaniferous  iron  is  opaque  and 
black,  with  a  semi-metallic  lustre. 

Chemically  it  may  be  a  combination  of  titanium  and 
peroxide  of  iron,  or  a  combination  of  titanic  acid  with 
protoxide  of  iron,  plus  a  variable  amount  of  the  peroxide. 
The  different  varieties  of  titaniferous  iron  depend  mainly 
upon  the  relative  proportions  of  iron  and  titanium  which 
they  contain.  When  heated  alone  before  the  blowpipe 
titaniferous  iron  is  infusible.  Heated  in  concentrated  sul- 
phuric acid  it  affords  a  blue  colouration  but  is  insoluble. 
When  pulverised  it  is,  however,  soluble  in  nitro-hydrochloric 
acid. 

When  occurring  in  microscopic  preparations  it  is  often 
difficult  to  distinguish  it  from  magnetite,  except  when  it 
affords  well-marked  rhombohedral  sections  (fig.  70),  It 
may,  however,  often  be  recognised  by  the  peculiar  greyish- 
FIG  o  wmte  alteration-product  which  is  often  developed 
within  it,  and  which  frequently  follows  definite 
crystallographic  directions.  When  this  alteration 
is  far  advanced,  merely  a  dark  skeleton  or  a  few 
dark  specks  of  the  unaltered  mineral  remain,  surrounded  by 


Hematite.  1 5  5 

the  white  decomposition  product.  The  precise  nature  of 
this  white  substance  has  not  yet  been  ascertained,  but  it  is 
generally  assumed  to  be  either  titanic  acid  or  some  silicate 
of  titanium. l 

Both  titaniferous  iron  and  magnetite  frequently  occur 
together  in  the  same  rock. 

HEMATITE. 

The  crystallised  variety,  specular  iron  or  ironglance, 
belongs  to  the  rhombohedral  system,  and  mostly  occurs  in 
six-sided,  thin,  tabular  crystals  in  which  the  basal  planes 
are  largely  developed,  while  the  boundaries  are  formed 
either  by  faces  of  the  rhombohedra  R  and  —  JR,  or  by  faces 
of  a  hexagonal  prism.  Crystals  of  this  kind  may  be  easily 
procured  by  dissolving  a  fragment  of  the  mineral  carnallite, 
when  the  residue  will  be  found  mainly  to  consist  of  beauti- 
fully-developed thin  tabular  crystals  of  specular  iron,  which 
are  translucent,  and  of  a  clear  red  or  orange-red  colour. 
Thicker  crystals  appear  black  or  iron-grey,  and,  as  in  some 
of  the  specimens  from  Elba,  show  beautiful  superficial  iri- 
descence. Sometimes  the  crystals  are  only  imperfectly  de- 
veloped, or  merely  form  irregularly-  shaped  scales.  In  this 
scaly  condition  it  is  spoken  of  as  iron-mica  or  micaceous  he- 
matite (Eisenglimmer).  Hematite  also  occurs  in  a  granular 
state,  sometimes  earthy  as  reddle,  while  reddish  stains  of 
ferric  oxide  are  of  common  occurrence  in  rocks,  especially  in 
those  which  have  undergone  weathering. 

The  botryoidal  or  mammillated  forms  of  hematite  mostly 
occur  in  pockets  or  cavities  in  rocks  into  which  they  have 
been  subsequently  introduced,  or  else  line  drusy  cavities,  but 
hematite  in  this  form  does  not  occur  as  a  common  rock 
constituent,  although  in  a  compact  and  massive  condition 
it  is  often  met  with  in  lodes.  In  some  cases,  however,  the 
massive  and  micaceous  forms  of  hematite  may  almost  of 

1  It  has  since  been  examined  and  described  by  Giimbel,  under  the 
name  of  leucoxene. 


! 


156  The  Rudiments  of  Petrology. 

themselves  be  regarded  as  rock  masses  ;  a  hill  in  the  state  of 
Missouri  (the  Pilot  Knob,  700  feet  high)  consisting  almost 
exclusively  of  hematite. 

Hematite  gives  a  blood-red  or  cherry-red  streak.  It  is 
feebly  magnetic.  Its  chemical  composition  is  Fe2O3  when 
pure,  but  it  is  often  rendered  impure  by  admixtures  of  sand, 
clay,  &c.  Before  the  blowpipe  it  is  infusible,  but  becomes 
black  and  strongly  magnetic  when  heated  in  the  reducing 
flame. 

Under  the  microscope  ;t  is  usually  seen  to  occur  in  irre- 
gular flecks  and  scales,  distinct  crystalline  forms  not  being 
of  common  occurrence  in  rocks. 

It  exhibits  no  dichroism,  and  shows  red  tints  of  various 
intensity  by  transmitted  light.  By  reflected  light  it  also 
usually  appears  red,  especially  when  in  an  earthy  or  finely 
granular  condition. 

LIMONITE. 

This  is  a  hydrated  peroxide  of  iron  having  the  formula 
2Fe2O3,  3H2O.  It  is  essentially  a  decomposition  product, 
resulting  from  the  alteration  of  protoxides,  or  of  anhydrous 
peroxides  of  iron,  which  have  previously  existed  as  consti- 
tuents of  other  minerals,  or  in  the  latter  case  sometimes 
simply  as  hematite  itself.  Limonite  occurs  in  stalactitic, 
mammillated,  pisolitic,  or  earthy,  conditions.  It  is  com- 
monly blackish-brown  or  yellowish-brown,  in  an  earthy  or 
ochreous  state  often  yellow.  The  streak  is  yellowish-brown. 
In  thin  sections  of  rocks  it  is  often  seen  to  occur,  forming 
pseudomorphs  after  crystals  of  various  ferruginous  silicates, 
and  as  irregularly-shaped  blotches.  It  appears  opaque 
under  the  microscope,  or  occasionally,  in  very  thin  sections, 
it  is  feebly  translucent,  and  of  a  brownish  colour. 

IRON  PYRITES. 

Crystallises  in  the  cubic  system,  the  most  common  form 
being  the  cube.  The  faces  of  the  crystals  are  frequently 


Iron  Pyrites.     Copper  Pyrites.  157 

striated,  the  striae  on  one  face  lying  at  right  angles  to  those) 
on  the  adjacent  faces.  Pyrites  also  occurs  massive,  in  no- 
dules which  have  internally  a  radiating  structure,  (many  of 
these  may  no  doubt  be  referred  to  marcasite),  while  in 
some  rocks  it  exists  in  a  granular  or  finely-disseminated 
state/sometimes  forming  pseudomorphs  after  other  minerals. 
/Fossils  are  at  times  entirely  replaced  by  pyrites.  It  is" 
mostly  of  a  pale  brass-yellow  colour,  gives  a  greenish  or 
brownish-black  streak  and  a  conchoidal  or  uneven  fracture. 
It  has  a  strong  metallic  lustre,  strikes  fire  with  steel,  and 
fuses  before  the  blowpipe  to  a  metallic  globule  which  is 
attractable  by  the  magnet.  When  heated  it  gives  off  sulphur. 
When  fused  with  carbonate  of  soda,  the  assay,  if  placed  on  a 
clean  silver  surface,  and  moistened  with  a  drop  of  water, 
produces  a  dark  stain  on  the  silver.  Its  chemical  com- 
position is  iron  =  467,  sulphur  =  53-3,  giving  the  formula 
FeS2. 

Under  the  microscope,  in  thin  sections  of  rocks,  pyrites 
appears  perfectly  opaque.  The  ground  surfaces  look  glis- 
tening and  yellowish  by  reflected  light,  and  this  partly  serves 
to  distinguish  it  from  magnetite. 

The  sections  are  those  resulting  from  cubes  or  dode- 
cahedra  sliced  in  various  directions,  except  in  cases  where 
the  mineral  is  pseudomorphous  after  some  other  mineral. 
Occasionally  pyrites  occurs  in  minute  elongated  rod-like 
forms. 

Marcasite  resembles  pyrites,  except  that  it  crystallises  in 
the  rhombic  system.  Twinning  is  common  in  this  species. 

COPPER  PYRITES  (CHALCOPYRITE). 
This  mineral  is  occasionally  met  with  in  rocks  such  as 
diabase,  some  granites,  gneiss,  argillaceous  schists,  &c. 
It  crystallises  in  the  tetragonal  system ;  the  crystals,  however, 
closely  approximating  to  cubic  forms.  It  usually  has  a 
deeper  yellow  colour  than  iron  pyrites,  from  which  it  may  be 
distinguished  by  its  inferior  hardness,  being  sectile,  while 


1 5  8  The  Rudiments  of  Petrology. 

iron  pyi  es  cannot  be  cut  with  a  knife.  Copper  pyrites 
does  noi  emit  sparks  when  struck  with  steel.  Before  the 
blowpipe  it  colours  the  borax  bead  blue  in  the  oxidising 
flame,  but  to  get  this  colouration  the  assay  should  not 
be  previously  reduced,  for,  if  so,  only  a  deep  green  co- 
louration will  be  procured.  The  blue  colour  is  probably 
due  to  sulphate  of  copper,  and  a  previous  roasting  of  the 
assay  of  course  expels  the  sulphur.  It  is  soluble  in  nitric 
acid,  with  the  exception  of  the  contained  sulphur,  forming 
a  green  solution  which  changes  to  a  deep  blue  on  the  addi- 
tion of  ammonia  in  excess. 

The  chemical  composition  of  copper  pyrites  is  copper 
=  32'5— 34-  Iron=  2975  —  31-25.  Sulphur  =  34  —36 
per  cent.  The  formula  is  Cu2  S,  Fe2  S3. 

Under  the  microscope  it  appears  opaque.  By  reflected 
light  it  shows  a  somewhat  metallic  lustre  on  ground  sur- 
faces, and  is  generally  rather  deeper  in  colour  than  iron 
pyrites,  but  not  much  reliance  can  be  placed  upon  this 
appearance,  and  its  presence  should  be  confirmed  by  che- 
mical examination. 

ZEOLITES. 

Want  of  space  precludes  more  than  a  brief  mention  of 
the  microscopic  characters  of  a  few  of  the  most  common 
zeolites.  They  may  all  of  them  be  regarded  as  alteration 
products,  and  in  all  probability  never  form  normal  con- 
stituents of  rocks.  They  usually  occur  either  lining  or 
completely  filling  cavities  in  vesicular  and  other  volcanic 
rocks,  and  also  occupy  fissures  and  small  cracks;  occasionally 
they  are  developed  in  crystals  of  other  minerals  which  have 
undergone  more  or  less  alteration.  They  often  occur  in 
spherical  crystalline  aggregates,  with  a  radiating  structure, 
in  which  case  they  exhibit  a  black  cross  when  examined 
between  crossed  Nicols,  the  arms  of  the  cross  coinciding 
with  the  planes  of  vibration  of  the  Nicols.  The  section 
may  be  horizontally  rotated  while  the  crossed  Nicols 


Natrolite.     Analcime.  159 

remain  stationary,  yet,  although  the  object  revel, es,  the 
dark  cross  does  not  move.  This  is  explained  by  G^oth  as 
being  due  to  the  principal  directions  of  vibration  of  the 
doubly-refracting  fibres  lying  parallel  and  at  right  angles  to 
their  longer  axes,  and  bearing  a  similar  relation  to  rays  which 
undergo  extinction  between  the  crossed  Nicols.  If  the 
analyser  be  turned  through  10°  or  20°  the  dark  cross 
becomes  somewhat  faint,  and  a  second  imperfectly  developed 
cross  appears,  which  makes  an  angle  of  5°  or  10°  with  the 
fixed  cross.  It  will  therefore  be  seen  that  it  travels  at  only 
half  the  rate  of  rotation  of  the  analyser.  When  the  analyser 
has  been  so  far  turned  that  the  two  Nicols  stand  parallel, 
the  first  cross  disappears  and  the  second  imperfect  cross 
attains  its  maximum  intensity.  This  phenomenon  is  met  with 
in  all  doubly-refracting,  radiate  crystalline  aggregates  ;  and, 
since  zeolites  frequently  occur  in  this  condition,  its  presence 
in  certain  rocks  often  suggests  that  such  aggregates  are 
zeolitic. 

Natrolite,  which  crystallises  in  the  rhombic  system,  pos- 
sesses weak  double  refraction,  and  polarises  in  vivid  colours. 
It  very  commonly  occurs  in  crystalline  aggregates,  which 
almost  invariably  have  a  radiate  structure,  and  then  show, 
especially  when  in  rounded  masses,  the  interference  figure 
characteristic  of  such  aggregates.  At  times,  also,  natrolite 
is  seen  filling  minute  fissures.  In  this  case  crystallisation 
commences  on  either  side  of  the  fissure,  and  the  crystals 
meet  in  the  middle,  their  termination  giving  rise  to  a  zig-zag 
median  line  which  divides  the  two  growths.  Nepheline 
crystals  at  times  become  partly  altered  into  natrolite,  a 
meshwork  of  little  prisms  of  natrolite,  in  some  instances, 
almost  completely  filling  the  crystal.1 

Analcime,  so  far  as  is  yet  known,  crystallises  in  the  cubic 
system,  but,  although  regarded  as  cubic,  it  exhibits  some 
rather  exceptional  optical  properties,  first  pointed  out  by 
Brewster,  and  subsequently  investigated  by  Descloizeaux, 

1  Rosenbusch,  Mik.  Physiog.  (Min.)  vol.  i.  p.  285.   Stuttgart,  1873. 


160  The  Rudiments  of  Petrology. 

Rosenbusch,  and  other  observers.  According  to  Des- 
cloizeaux,  sections  cut  parallel  to  any  one  of  the  faces 
of  the  cube,  when  viewed  in  the  direction  of  one  of  the 
axes  by  parallel  polarised  light,  appears  between  crossed 
Nicols  perfectly  dark  in  the  direction  of  the  two  other 
axes,  while,  in  the  direction  of  the  diagonals  of  the  cube- 
face,  a  faint  bluish,  distorted  cross  appears.  Analcime  is 
seldom  or  never  a  normal  constituent  of  rocks.  Tschermak, 
however,  regards  it  as  an  essential  component  of  the  rock 
which  he  terms  teschenit,  which  consists  of  plagioclase, 
hornblende,  analcime,  magnetite,  biotite,  and  apatite.  In  a 
leucitophyr  from  Rothweil,  near  the  Kaiserstuhl,  analcime 
occurs  pseudomorphous  after  leucite  ;  at  .all  events,  the 
leucite  crystals  contain  fibrous  and  granular  aggregates  of 
analcime,  which  at  times  almost  totally  replace  them. 

Heulandite  has  been  observed  to  contain  various  micro- 
scopic inclosures  such  as  minute  orange-yellow  coloured 
acicular  crystals,  irregularly- shaped  or  round  granules  and 
flecks  of  a  reddish-yellow  mineral,  and,  in  one  specimen, 
from  the  Faroe  Isles,  Rosenbusch  noted  the  occurrence  of 
innumerable  perfectly-developed  microscopic  quartz  crys- 
tals. The  colour  of  heulandite  is  due  to  the  reddish  and 
orange-yellow  inclosures  just  alluded  to.  They  have  been 
regarded  as  gothite,  limonite,  or  hematite.  Zirkel  considers 
that  they  are  hematite. 

Chabasite. — This  mineral  appears  from  numerous  obser- 
vations always  to  be  devoid  of  fluid  inclosures.  Micro- 
scopic envelopments  of  quartz  have  been  met  with  in 
chabasite. 

For  further  particulars  respecting  the  large  family  of 
zeolites  the  student  is  referred  to  the  various  manuals  and 
text-books  of  mineralogy. 

CRYSTALLITES. 

Under  this  head  may  be  grouped  a  vast  number  of 
purely  microscopic  bodies,  which,  in  their  progressive  de- 


Crystallites.  161 

velopment,  represent  the  various  forms  and  conditions  of 
mineral  matter,  from  its  departure  from  an  amorphous  state, 
to  one  of  crystallographic  completeness,  such  as  may  be 
correlated,  if  not  identified,  with  the  crystals  of  recognised 
mineral  species.  The  forms  belonging  to  the  highest  stage 
of  this  microscopic  development  are  spoken  of  as  rnicroliths, 
and  they  frequently  present  crystal  faces  sufficiently  distinct 
to  admit  of  goniometric  measurements,  and  optical  characters 
well  enough  denned  to  permit  their  correlation  with  recog- 
nised minerals.  The  less  perfectly  developed  crystallites 
cannot  however  be  referred  to  any  particular  species,  and 
hence  has  arisen  the  necessity  for  the  employment  of  various 
terms,  more  or  less  vague,  and  each  of  them  embracing  a 
vast  multitude  of  different  forms,  but  convenient,  because 
indicative  of  structural  types.  Doubtless,  as  knowledge 
increases,  these  terms  will  give  place  to  better  ones  with 
more  precise  significations,  and  the  progressive  developments 
which  these  minute  forms  display  will  be  properly  worked 
out,  and  afford  a  key  to  the  important  subject  of  crystallo- 
genesis. 

The  crystallites  may  be  ranged  in  a  descending  series  as 
follows : 

Microliths. 

Crystalloids. 

Trichites. 

Globulites. 

The  globulites  represent  the  most  embryonic  stage  of 
crystallogenesis,  the  most  rudimentary  change  effected  in 
amorphous  matter.  They  are  spherical  in  form,  and  by  their 
coalescence  give  rise  to  variously  shaped  groups,  according 
to  the  number  of  individual  globulites  which  enter  into  their 
composition.  Sometimes  they  arrange  themselves  in  strings, 
and  into  other  systems  of  disposition,  implying  more  or  less 
symmetrical  grouping.  They  usually  show  a  central  speck 
or  nucleus,  and  at  times  display  concentric  markings  and 
indications  of  a  radiate  structure. 

M 


1 62  The  Rudiments  of  Petrology. 

Trichites  (from  fy><£,  a  hair)  are  minute  elongated  bodies 
resembling  small  hairs  or  fibres ;  sometimes  they  are  straight, 
sometimes  they  cross  one  another  in  a  more  or  less  regular 
manner  ;  at  others  they  appear  bent  in  zigzags,  or  are  curi- 
ously twisted,  while  occasionally  a  number  of  trichites  emanate 
from  a  central  granule  around  which  they  radiate  or  twirl 
like  whip-lashes.  Some  trichites  show  regular  or  interrupted 
lines  of  granules  attached  to  them,  forming  rows  like  beads 
either  upon  one  or  upon  both  sides  of  the  trichite. 

The  crystallites  proper  and  crystalloids  exhibit  in  many 
instances  a  much  higher  development,  being  bounded  by 
curved  or  by  straight  lines,  and  often  assuming  crucial  or 

stellate  forms,  which  appear 
to  result  from  the  symmetri- 
cal grouping  of  individual 
crystallites.  The  crystalloids 
especially  exhibit  consider- 
able complexity  of  internal 
structure,  while  in  external 
form  they  often  approximate 
to  crystals  of  recognised 
minerals.  Some  of  them 
indeed  show  so  close  a 

resemblance  to  true  crystals  that  one  cannot  help  feeling 
impressed  with  the  significance  of  their  internal  structure 
when  contrasted  with  that  of  larger  crystals. 

The  accompanying  figures  convey  a  far  better  idea  than 
any  description  could  of  the  forms  which  these  minute  bodies 
present. 

Microliths. — These  again  show  a  more  complete  phase 
of  development  than  the  preceding  forms.  They  are  some- 
times very  imperfectly  developed,  but  in  all  cases  it  is  gene- 
rally considered  that  they  exhibit  a  nearer  approximation  to 
true  crystals.  They  very  commonly  display  double  refraction, 
occasionally  show  hemitropy,  and  frequently  present  suf- 
ficiently well-developed  faces  to  enable  the  observer  to 


Microliths.  163 

measure  their  relative  inclination.  In  some  of  the  larger 
microliths  dichroism  may  now  and  then  be  detected.  It  is 
therefore  possible  at  times  to  determine  with  some  precision 
the  species  to  which  a  microlith  belongs.  Occasionally 
crystals  are  to  be  met  with  which  are  visibly  built  up  of 
microliths,  as  in  the  case  of  the  hornblende  crystal,  fig.  72, 
which  is  copied  from  a  woodcut  in  Zirkel's 

FIG.  72. 

'  Mikroskopische  Beschaffenheit  der  Miner- 
alien  und  Gesteine.'  Microliths  are  to  be 
found  in  most  eruptive  rocks,  and  in  many 
metamorphosed  sedimentary  deposits.  Glo- 
bulites,  trichites,  crystallites,  and  crystalloids 
may  best  be  studied  in  sections  of  vitreous  ., 
rocks  such  as  obsidians,  pitchstones,  and 
perlites,  also  in  artificially  formed  glasses  and 
slags.  Streams  of  microliths  may  commonly 
be  seen  under  the  microscope  in  sections 
of  pitchstone  and  perlite.  They  often  lie  with  their 
longest  axes  in  one  direction,  which  represents  the  direction 
of  flow  in  the  once  viscid  mass,  for  we  not  merely  see  micro- 
liths but  also  strings  of  vitreous  matter,  spherulites,  &c., 
elongated  and  drawn  out  in  the  same  direction.  The 
microliths  sweep  in  curves  round  any  large  crystals  or  frag- 
ments which  may  chance  to  lie  in  their  course,  and  seem  to 
have  behaved  just  as  planks  or  sticks  do  when  floating  down 
a  stream.  These  appearances  in  a  rock  are  spoken  of  as 
fluxion  structure  or  fluidal  structure. 

The  development  of  microliths  is  one  of  the  causes  of 
devitrification  in  glassy  rocks  and  in  artificial  glass.  Micro- 
liths also  occur  as  products  of  alteration,  frequently  filling  or 
partially  filling  the  interior  of  crystals  which  are  undergoing 
decomposition. 

For  further  information  upon  the  microscopic  characters  of 
crystallites,  both  of  natural  occurrence  and  of  artificial  for- 
mation, the  reader  is  referred  to  '  Die  Krystalliten J  by  the 
late  Hermann  Vogelsang.  Bonn,  1875. 

M  2 


164  T/ie  Rudiments  of  Petrology, 

FLUID  INCLOSURES,  &c. 

When  salts  are  allowed  to  crystallise  from  a  saturated 
solution,  it  is  by  no  means  uncommon  to  find  that  the 
crystals,  in  the  course  of  their  formation,  shut  in  small 
portions  of  the  mother-liquor ;  and  should  the  temperature 
at  which  the  crystals  form  be  a  moderately  high  one,  the 
imprisoned  fluid  will,  upon  cooling,  diminish  in  volume,  so 
that  a  vacuity  in  the  form  of  a  bubble  will  also  be  seen  to 
occupy  a  portion  of  the  cavity  originally  filled  by  the  liquid. 
The  relative  dimensions  of  these  bubbles  to  the  cavities 

which    contain   them    have 

FIG.  73. 

been  carefully  studied  by 
Sorby,  Renard,  Phillips, 
Ward,  and  other  observers. 
The  cavities  which  contain 
these  fluids  are  of  very  vari- 
able form  (fig.  73).  Occa- 
sionally they  are  so  large  as 
to  be  distinctly  visible  to 
the  naked  eye,  but  usually 

they  are  of  quite  microscopic  dimensions.  In  the  excep- 
tionally large  ones  the  bubble  may  be  seen  to  move  to 
different  parts  of  the  cavity  by  merely  turning  the  crystal 
about  in  the  hand.  In  the  microscopic  cavities  the 
bubble  can  be  made  to  move  and  the  liquid  to  expand 
by  the  application  of  heat.  This  may  be  effected  either 
by  means  of  a  voltaic  current  or  by  a  blast  of  heated 
air.1  The  cavities  containing  air  and  gases,  which  are 
sometimes  met  with  in  crystals,  present  strong,  dark  out- 
lines, which  serve  to  distinguish  them  from  those  containing 
fluids,  while  the  differences  in  the  refractive  indexes  of  their 
contents  also  serve  as  another  distinction.  Furthermore, 
although  some  cavities  occur  completely  filled  with  liquid, 

1  W.  N.  Hartley,  '  On  Identification  of  Liquid  Carbonic  Acid  in 
Mineral  Cavities,'  Trans.  Royal  Mic.  Sac.,  vol.  xv.  p.  173,  1876. 


Glass  and  Stone  Inclosures.  165 

still  the  presence  of  movable  bubbles  in  most  of  the  fluid- 
containing  cavities  at  once  affords  a  means  of  distinguish- 
ing them.  In  some  microscopic  inclosures  of  fluid  very 
minute  bubbles,  which  have  a  spontaneous  motion,  may  be 
seen  under  high  powers. 

The  liquids  usually  contained  in  such  cavities  are  water, 
liquid  carbonic  acid,  and  aqueous  solutions  of  salts,  fre- 
quently of  chloride  of  sodium;  and  occasionally  cavities  may 
be  seen  in  quartz  which,  besides  the  liquids  and  bubbles, 
contain  minute  cubic  crystals  of  rock  salt  (fig.  74).1 

Glass  Inclosures  are  of  common  occur- 
rence in  the  minerals  which  are  met  with 
in  vitreous  rocks,  or  in  rocks  which 
contain  a  certain  amount  of  interstitial 
glassy  matter.  They  are  spherical,  sphe- 
roidal, fusiform,  or  of  very  irregular  shape, 
or  else  they  assume  definite  crystallogra- 
phic  forms,  corresponding  as  a  rule  with 
that  of  the  crystal  in  which  they  occur. 
Such  forms  may  be  regarded  as  negative 


crystals.  They  either  appear  as  singly  refracting  matter,  or, 
when  more  or  less  devitrified,  as  doubly  refracting.  In  the 
latter  case  they  may  be  devitrified  either  by  the  development 
of  crystalline  granules  or  of  microliths.  Glass  inclosures 
frequently  contain  bubbles,  but  these  bubbles  are  fixed, 
and  do  not  change  their  position  when  the  section  is 
heated. 

Stone  Inclosures  are  analogous  to  the  foregoing,  except 
that  they  consist  of  portions  of  a  rock's  magma  which  has  a 
crystalline  and  not  a  vitreous  character. 

In  the  cases  both  of  glass  and  stone  inclosures  small 
portions  of  the  matrix  have  been  taken  up  while  still  in  a 
fluid  or  pasty  condition  by  the  crystals  in  which  they  occur, 

1  Vide  Memoirs  sur  les  Roches  dites  Plutoniennes  de  la  Belgique,  De 
la  Vallee  Poussin  et  A.  Renard.  Bruxelles,  1876.  Also  'The  Eruptive 
Rocks  of  Brent  Tor  and  its  Neighbourhood,'  Memoirs  of  the  Geological 
Survey  of  England  and  Wales,  F.  Rutley.  London,  1878. 


1 66  The  Rudiments  of  Petrology. 

and  the  crystal  having  developed  itself  before  the  solidifica- 
tion of  the  surrounding  magma,  these  small  portions  have 
been  shut  off  and  imprisoned.  Sometimes  the  severance  of 
the  little  mass  of  glass  or  other  matrix  has  not  been  perfectly 
effected,  and  it  merely  appears  as  a  small  pocket  with  a 
constricted  neck,  which  opens  out  on  the  margin  of  the 
crystal.  Such  partial  inclosures  may  frequently  be  seen  in 
the  quartz  of  quartz-porphyries  and  quartz-trachytes. 

PROVISIONAL  NAMES  APPLIED  TO  MINERALS. 

The  following  are  terms  used  to  designate  provisionally 
certain  substances  which  are  sometimes  met  with  in  thin 
microscopic  sections  of  rocks,  and  which,  from  occurring 
only  in  very  minute  quantities  difficult  of  isolation,  it  has 
not  as  yet  been  possible  to  analyse.  Their  precise  che- 
mical constitution  and  mineralogical  affinities  are,  therefore, 
undetermined;  and,  to  avoid  erroneous  descriptions  of  them, 
certain  terms  have  been  coined  which  merely  imply  sub- 
stances which  present  certain  microscopical  appearances, 
and  whose  mineralogical  'characters  may  vary  more  or  less, 
and  may  embrace  several  distinct  mineral-species  under 
each  .term. 

Viridite  includes  mineral  matter  which  is  probably  re- 
ferable to  different  varieties  of  chlorite  and  serpentine.  It 
appears  under  the  microscope  in  the  form  of  translucent 
green  matter,  either  in  little  scales,  or  fibrous  aggregates. 
It  may  always  be  regarded  as  a  product  of  decomposition, 
and  frequently  represents  the  alteration  of  such  minerals  as 
hornblende,  augite,  olivine,  &c.  It  is  probably  a  silicate  of 
magnesia  and  protoxide  of  iron. 

Opatite  is  the  term  applied  to  perfectly  opaque,  black, 
amorphous,  microscopic  granules,  patches,  or  scales.  It  is 
usually  present  in  rocks  which  contain  magnetite;  frequently 
it  forms  pseudomorphs  after  other  minerals.  It  is  regarded 
by  Zirkel  as  representing  earthy  silicates,  possibly  allied  to 
micas,  and  amorphous  metallic  oxides,  especially  hydroxides 


xv. 

Felsitic  Matter.          /  j .  167 

and  oxides  of  titanium  and  manganese/  /In  soto'ev  cases 
it  may  be  amorphous  magnetite,  or  carbonaceous  matter 
graphite,  &c. 

Ferrite  is  amorphous  red,  brown,  or  yellow  earth/ matter 
which  is  often  pseudomorphous  after  ferruginous  minerals/ 
Chemically  it  most  likely  represents  hydrous  or  anhydrous^ 
oxides  of  iron,  but  the  different  kinds  cannot  be  referreoT~ 
with  any  certainty  to  definite  mineral  species. 

FELSITIC  MATTER. 

This  substance,  which  is  of  such  common  occurrence  in 
many  rocks,  and,  in  some,  constitutes  a  very  large  proportion, 
forming  the  groundmass  of  quartz-porphyries  and  many 
other  porphyritic  rocks,  and  often  representing,  in  the  rhyo- 
litic  series,  the  devitrification  of  glassy  matter,  has  hitherto 
been  described  in  a  more  or  less  vague  manner  by  numerous 
observers.  The  student  has  consequently  been  left  in  a 
state  of  considerable  doubt  as  to  what  felsitic  matter  really 
is,  and,  as  a  rule,  the  more  he  has  read  on  the  subject,  the 
less  able  has  he  been  to  fix  any  satisfactory  definition  for 
the  term. 

A  masterly  account  of  the  various  opinions  which  have 
been  put  forward  on  this  subject  will  be  found  in  Rosen- 
busch's  '  Mikroskopische  Physiographic  der  massigen  Ge- 
steine.'  Stuttgart,  1877,  p.  60  et  seq.  In  this  place  it  will 
suffice  to  give  the  conclusions  arrived  at  by  Prof.  Rosen- 
busch,  since,  although  they  represent  in  part  the  views  of 
Prof.  Zirkel,  they  seem  to  meet  most  of  the  objections  to 
which  other  definitions  are  open,  and  possess  a  precision 
hitherto  wanting  in  most  descriptions  of  these  difficultly 
determinable  substances. 

To  begin  with  ;  these  substances  which  cannot  be  pro- 
perly investigated,  except  under  high  magnifying  powers, 
may  be  resolved  microscopically  either  into  a  thoroughly 
crystalline  aggregate,  or  into  homogeneous,  amorphous 
matter. 


1 68  The  Rudiments  of  Petrology. 

The  former  is  designated  groimdmass  by  Vogelsang  and 
Rosenbusch ;  and  the  latter  magma. 

Zirkel  employs  the  name  basis  for  the  amorphous  sub- 
stance, or  '  unindividualised  ground-paste/  as  he  terms  it, 
and  Rosenbusch  also  adopts  the  term  basis. 

Zirkel's  definition  of  a  micro-felsitic  basis  is:  'that  it  is 
amorphous,  that  it  shows,  in  thin  sections,  no  independent 
contours.     Its  boundaries  are  moulded  upon  the  forms  of 
the  crystalline  constituents,  and  it  forms  roundish  creeks  or 
inlets  in  the  latter.    Its  true  nature  is  variable,  and  not  easy 
to  render  in  words.     It  represents  a  devitrification  product 
in  which,  indeed,  a  hyaline  aspect  is  utterly  wanting,  but 
which,  on  the  other  hand,  is  not  separable  into  true  indi- 
vidualised parts.     It  generally  consists  of  quite  indistinct, 
often  half-fluxed  granules,  or  indistinct  fibres  which  consti- 
tute  the   micro-felsitic  mass.     Between   crossed  Nicols  it 
becomes,  in  its  typical  development,  perfectly  dark,  but  also, 
at  times,  transmits  a  very  feeble,  fluctuating  light.  The  little 
fibres  and  granules  often  show  a  decided  or  a  rough  ten- 
dency to  radial  arrangement.     In  thin  section  micro-felsitic 
matter  appears  very  clear,  and  either  light-greyish,  yellowish, 
reddish,  or  quite  colourless.    It  is  often  speckled  with  little 
dark  granules  which  in  certain  spots  show  a  crude  radial 
arrangement,  or  it  may  be  sprinkled  with  brownish-yellow 
and  brownish-red  granules  of  a  ferruginous  mineral. 

An  ultimate  glass  magma  may  be  present  in  many  micro- 
felsitic  masses,  although  not  clearly  to  be  recognised  as 
such.  Experience  shows  that  weathered  felspars  may  be 
represented  by  micro-felsitic  matter. ! 

Rosenbusch  states  that  in  many  cases  felsite,  or  the 
groundmass  of  porphyries,  consists  of  a  microscopically 
fine-grained  aggregate,  formed  of  minerals  which  can  be 
identified  with  those  constituting  granitic  rocks,  often  in  the 
same  combinations  as  those  in  which  they  occur  in  rocks 

1  Mikroskop.  Beschaff.  d.  Min.  in  Gest.,  Zirkel.  Leipzig,  1873, 
p.  280. 


Felsitic  Matter.  169 

of  the  granitic  group.  One  or  other  of  these  minerals  is 
often  absent,  and  of  these  mica  is  the  one  which  is  generally 
missing.  So  long  as  the  granules  of  such  aggregates,  which 
differ  in  no  essential  respect  from  many  vein-granites,  or  the 
groundmasses  of  many  granite-porphyries,  are  not  of  too 
minute  dimensions,  one  can  recognise  the  mosaic-like 
aggregate  polarisation  and  the  sharp  boundaries  of  the  indi- 
vidual crystalline  grains.  Diminution  in  the  size  of  the 
grains  naturally  renders  the  recognition  of  the  individual 
particles  more  difficult,  and  often  impossible.  The  individual 
granules  do  not  merely  lie  side  by  side,  but  also  in  various 
planes  one  over  another,  and  the  various  refractions,  reflec- 
tions, and  interferences  which  ensue  from  these  overlaps  tend 
to  render  any  deductions  concerning  the  optical  characters 
of  the  constituent  granules  highly  untrustworthy,  and  give 
rise  to  the  generally  vague  transmission  of  light  which  cha- 
racterises these  aggregates  when  they  are  examined  between 
crossed  Nicols. 

Rosenbusch  goes  on  to  state  that  if  we  accept  Groth's 
definition  of  a  crystal  as  a  compact  body  in  which  the  elas- 
ticities differ  in  different  directions,  and,  if  we  furthermore 
allow  that  external  boundaries  are  immaterial  so  far  as  the 
foregoing  definition  extends,  it  follows  that  if  double  refrac- 
tion be  the  result  of  any  mechanical  conditions  of  tension, 
or  strain,  the  expression  '  non-individualised,'  used  by  some 
authors  in  reference  to  micro-felsitic  matter,  is  either  mean- 
ingless, or  that  it  indicates,  at  best,  that  external  form  does 
not  entail  special  internal  conditions. 

From  this  point  of  view  Rosenbusch  designates  as 
crystalline  all  those  parts  of  felsites  which  are  doubly  refract- 
ing, so  long  as  it  cannot  be  demonstrated  that  their  aniso- 
tropy  is  in  any  way  the  result  of  anything  resembling 
conditions  of  strain  which  are  not  related  to  molecular 
structure. 

Basing  his  nomenclature  upon  these  considerations, 
Rosenbusch  describes  those  parts  of  porphyritic  ground- 


1 70  The  Rudiments  of  Petrology. 

masses  which  are  aggregates  of  mineralogically-recognisable 
elements  as  micro-crystalline,  while  those  parts  which  are 
simply  crystalline  aggregates,  without  any  definite  character 
being  discernible  in  the  constituent  particles,  he  terms 
crypto-crystalline. 

Those  portions  of  porphyritic  groundmasses  in  which  no 
double  refraction  can  be  recognised  must  be  regarded  as 
amorphous,  although,  as  Rosenbusch  remarks,  that,  ex- 
cluding isometric  crystals,  there  are  those  belonging  to 
other  systems  in  which  the  elasticity-differences  in  some 
directions  become  too  insignificant  to  afford  any  perceptible 
phenomena  o/  double  refraction  when  thin  sections  of  them 
are  examined.  He  also  cites  certain  alteration-products 
after  pyroxene  and  amphibole,  in  which  their  anisotropic 
character  can  only  be  distinguished  by  their  pleochroism. 
In  very  many  cases  micro-  or  crypto-crystalline  matter 
contains  an  intimate  admixture  of  fine  films,  stripes,  and 
flecks  of  a  perfectly  structureless  and  almost  invariably 
colourless  substance  which  remains  dark  in  all  positions 
between  crossed  Nicols.  It  may  be  absolutely  homoge- 
neous, or  it  may  contain  excessively  minute  granules  and 
trichitic  bodies  of  various  kinds.  This  substance  Rosen- 
busch designates  glass  or  glassy-basis.  The  condition  in 
which  it  contains  the  granules  and  trichites  he  regards  as  a 
phase  of  devitrification.  In  most  instances  this  impure  or 
devitrified  matter  is  opaque,  or  so  feebly  translucent,  and 
occurs  in  such  minute  films  or  grains,  that  a  determination  of 
its  isotropic  or  anisotropic  character  is  seldom  possible. 

Instead  of  a  true  glassy-basis,  matter  of  a  somewhat 
different  kind  is  very  often  present,  forming  excessively  thin 
films  which  appear  interwoven  with  the  micro-  or  crypto- 
crystalline  aggregates.  This  substance  is  perfectly  isotropic, 
colourless,  greyish,  yellowish,  or  brownish,  but,  unlike  true 
glass,  it  is  not  structureless,  but  appears  to  be  made  up  of 
extremely  minute  scales,  fibres,  granules,  or  aggregates  of 
granules,  together  with  other  developed  forms  and  interstitial 


Cleavages. 


171 


matter.  It  differs  also  from  micro-  and  crypto-crystalline 
aggregates  in  its  want  of  any  action  upon  polarised  light.  This 
substance  is  the  micro-felsite  or  micro -felsitic  basis  of  Rosen  - 
busch.  It  is  not  micro-  or  crypto-crystalline,  and  it  is  not 
amorphous  in  the  sense  in  which  those  terms  are  employed. 

Rosenbusch  adds  that  it  yet  remains  to  be  shown  whether 
micro-felsitic  matter  is  inert  upon  polarised  light,  owing  to 
exceptional  conditions  of  tension,  or  whether  it  should  be 
regarded  as  a  fibrous,  scaly,  or  granular  glass,  or  as  some- 
thing else. 

The  observations  of  Leopold  von  Buch,  Delesse,  Stelz- 
ner,  Wolff,  Vogelsang,  Allport,  Kalkowsky,  and  other  writers, 
are  commented  upon  in  the  review  of  this  subject,  which 
Rosenbusch  gives  in  the  work  from  which  these  statements 
have  been  extracted.1 


TABLE  SHOWING  THE  CLEAVAGES  OF  THE  MOST  COMMON 
ROCK-FORMING  MINERALS. 

Cubic  System. 

Dodecahedron 


Garnet 
Hauyne 
Nosean 
Sodalite 


very  imperfect 
rather  perfect 


Tetragonal  System. 


Prism(2nd  order) 

Prism  (ist  order) 

Basis 

Pyramid(ist  ord.) 

00   P   00 

OOP 

oP 

P 

Leucite 

most  imperf. 

— 

most  imperf. 



Scapolite 

rather  perfect 

less  distinct 

— 



Zircon 

— 

imperfect 

— 

imperfect 

Melilite 

— 

— 

rather  perfect 

1  Miltroskopische  Physiographic.    Stuttgart,  1877,  bd.  ii.  p.  60  et  seq. 


172 


The  Rudiments  of  Petrology. 
Hexagonal  System. 


Rhombohedron 

Prism 

Basis 

R 

00    P 

o  P 

Biotite 

— 



— 

highly  perf. 

Apatite 

— 

— 

imperfect 

imperfect 

Tourmaline 

very  imperf. 

—      . 

f  ooP2  very) 
1    imperf.     J 

— 

Calcspar 

very  perfect 

— 

— 

— 

Nepheline 

— 

— 

imperfect 

imperfect 

/  traces  have  \ 

Quartz  l 

most  imperf. 

— 

j    been   ob-  I 

_- 

(  served       ) 

Rhombic  System. 

Brachy-pina- 

Macro-pina- 

koid 

koid 

Prism 

Basis 

00   P  00 

00   P   00 

00    P 

oP 

Olivine 

rather  distinct 

very  imperf. 





Enstatite 

imperfect 

— 

distinct 



Bronzite 

very  perfect 

traces 

imperfect 

-  — 

Hypersthene 

M 

very  imperf. 

distinct 

— 

Andalusite 

traces 

traces 

not  very  dist. 

— 

Muscovite 

— 

— 

imperfect 

highly  perfect 

Monoclinic  System. 

Ortho-pinakoid 

Clino-pinakoid 

Prism 

Basis 

oo  -E  oo 

00  £  00 

OOP 

oP 

Orthoclase 

— 

very  perfect 

very  imperf. 

very  perfect 

Augite 

imperfect 

imperfect 

j  more      or  ) 
1  less  perf.   J 

— 

Diallage 

perfect 

5) 

rather  perf. 

— 

Hornblende 

very  imperf. 

very  imperf. 

very  perfect 

— 

Epidote 

perfect 

— 

— 

very  perfect 

TricUnic  System. 

Brachy-pina- 

Macro-pina- 
koid 

Prism 

Basis 

00    P   00 

00    P  00 

00    P 

oP 

Labradorite 

rather  perfect 

— 

— 

very  perfect 

Oligoclase 



i  hemipr.  im- 
j    perf.,    also 

\  perfect 

I   pr.  imperf. 

) 

Anorthite 

perfect 

— 

— 

»» 

Albite 

f  hemipr.  im- 

}         2 

1    perf. 

1  Cleavage  in  quartz  is  rare.        2  Also  tetarto-pyram.  imperfect. 


Cleavages. 


173 


PRISM     S7Q5' 


PLATE  II. 
PRISM    87°  5' 


PRISM  024° 30' 


HORNBLENDE 
PRISM      87°  5' 
CLEAVAGE  124°  30  f 


CO  P    GO 

HYPERSTHENE 


U  RALITE 


CLEAVAGES. 
MMMR   Highly  perfect.  — — —  Perfect. 

-  Imperfect.  -  Very  imperfect. 

The  observer  is  supposed  to  be  looking  down  directly  on  the  basal 
planes  of  the  crystals. 

Except  in  fig.  f,  the  angles  of  the  prismatic  cleavage  correspond  with 
those  of  the  prisms. 

Figs,  a,  b,  c,  and  f  represent  monoclinic  crystals.  Figs,  d  and  e 
represent  rhombic  crystals. 


174 


PART   II. 
DESCRIPTIVE  PETROLOGY. 

CHAPTER  XL 

THE   CLASSIFICATION    OF  ROCKS. 

THE  classification  of  rocks  involves  considerable  difficulty, 
and  no  scheme  has  yet  been  propounded  which  is  not  more  or 
less  open  to  objection.  Our  knowledge  is  not  at  present 
extensive  enough  to  enable  us  to  speak  with  certainty 
regarding  the  origin  of  all  the  different  rocks  with  which  we 
are  acquainted,  and  we  are  not  as  yet  in  a  position  to  assert 
how  far  the  mineral  constitution  and  the  physical  charac- 
ters of  rocks  afford  clues  to  their  origin.  The  following 
points  have  to  be  considered  in  framing  a  petrological 
classification : — 

(i.)  The  chemical  composition  of  the  rocks. 

(ii.)  Their  mineral  constitution. 

(iii.)  Their  physical  characters. 

(iv.)  Their  mode  of  occurrence. 

(v.)  Their  order  of  sequence  in  time. 

The  chemical  examination  of  a  rock  shows  us  what 
elementary  substances  enter  into  its  composition,  and  may 
afford  some  clue  to  its  mineral  constitution  and  to  its 
origin. 

The  mineralogical  and  physical  examinations  teach  us 
how  those  elementary  substances  have  combined,  and, 


Classification  of  Rocks.  175 

in  some  cases,  the  conditions  under  which  those  combina- 
tions have  been  effected,  the  various  minerals  which  enter 
into  the  composition  of  the  rock,  the  crystal! ographic  and 
other  physical  peculiarities  which  the  component  minerals 
present,  the  relative  order  in  which  those  minerals  have 
sometimes  crystallised,  the  arrangement,  if  any,  which  the 
individual  crystals,  granules,  scales,  or  fragments  observe 
towards  each  other,  and  the  general  state  of  aggregation  of 
the  crystals  or  mineral  particles  of  which  the  rock  is 
composed. 

The  microscope,  furthermore,  affords  the  means  of  ex- 
tending these  investigations  by  enabling  the  observer  to  see 
structural  peculiarities  which  unassisted  vision  fails  to 
detect. 

The  following  classification  has  been  framed  for  the 
purpose  of  bringing  certain  important  typical  rocks  promi- 
nently before  the  student's  notice,  these  type-rocks  consti- 
tuting, as  it  were,  the  nuclei  of  their  respective  groups. 
Since  the  groups  of  each  class  merge  into  one  another  more 
or  less  in  mineral  constitution,  no  sharp  boundary  lines  can 
be  drawn  between  them ;  the  type-rocks  of  the  different  groups 
therefore  serve  as  milestones  by  means  of  which  the  student 
may  ascertain  in  what  part  of  the  great  series  to  class  any 
particular  rock  ;  the  types  holding  a  relation  to  the  whole 
petrological  series  somewhat  analogous  to  that  which  Frauen- 
hofer's  lines  bear  to  the  spectrum. 

CLASSIFICATION  OF  ROCKS. 

ERUPTIVE  ROCKS. 

I.   Vitreous. 

Obsidian     \ 

Pumice        }•  including  hyaline  rhyolite. 

Perlite 

Pitchstone 

Tachylyte. 


176  Descriptive  Petrology. 

II.  Crystalline. 

*  /-Granite    group 

Felstone      „ 

Syenite        „ 

Trachyte  '  „   including  rhyolite  proper. 

Phonolite    „ 
A.  Typical  groups  \  Andesite     „ 

Porphyrite  „ 

Diorite  „>  included  under  the  old 
-Diabase  „[  term  'greenstone'  in 
-•Gabbro  „[  its  original  and  broadest 
-Basalt  „ )  signification. 


B.  Rocks  of  exceptional  mineral  constitution. 

III.   Volcanic  Ejectamenta. 
IV.  Altered  Eruptive  Rocks. 

SEDIMENTARY  ROCKS. 
I.  Normal  Series. 

Arenaceous  group  (sands). 
Argillaceous  „  (clays). 
Calcareous  „  (limestones). 

II.  Altered  Series. 

A.  With  no  apparent  crystallisation. 

B.  With  sporadic  crystallisation. 

C.  Crystalline  I  ^  ^on- foliated. 

[o.  foliated  and  schistose. 

III.  Coarse  Fragmental Series. 

Breccias  and  conglomerates. 

IV.  Tufas  and  Sinters. 

V.  Mineral  Deposits  consisting  Rock-Masses. 


Vitreous  Rocks.  177 


ERUPTIVE   ROCKS. 

CLASS  I. — VITREOUS  ROCKS. 

The  vitreous  rocks  are  characterised  by  their  generally 
homogeneous  aspect,  their  more  or  less  glassy  lustre  (which, 
however,  is  sometimes  only  feebly  glassy,  greasy,  or  dull 
when  the  rock  is  partially  devitrified),  by  their  conchoidal 
fracture,  and,  optically,  by  the  single  refraction  which  they 
exhibit,  except  when  more  or  less  crystalline  structure  has 
supervened.  They  may,  like  the  crystalline  eruptive  rocks, 
be  divided  into  two  sub-classes,  the  highly- silicated  or 
acid  (those  containing  over  60  per  cent,  of  silica),  and  the 
basic  (or  those  which  contain  less  than  60  percent).  Some 
of  those  usually  occurring  in  the  former  sub-class  vary 
somewhat  in  the  amount  of  silica  which  they  contain,  and  at 
times  appear  to  belong  rather  to  the  basic  sub-class  j  the 
pumice  from  some  localities,  for  example,  having  less  than 
50  per  cent,  of  silica,  while  that  from  others  contains  over 
62  per  cent. 

The  vitreous  rocks  may  be  conveniently  arranged  in  the 
following  order : — 

I.  Containing  over  60  per  cent.  SiO2 : — 

Obsidian. 

Pitchstone.  Pumice. 

Perlite. 

II.  Containing  less  than  60  per  cent.  SiO2 : — 

Tachylyte.  Pumice. 

The  vitreous  rocks  of  the  first  or  highly-silicated  sub- 
class closely  resemble  the  liparites,  trachytes,  andesites,  and 
other  highly-silicated  eruptive  rocks  in  their  chemical  com- 
position, while  the  minerals  which  are  developed  in  many  of 
them  also  imply  a  similarly  close  relationship.  So  close, 
indeed,  is  this  relation  that  some  petrologists  include 
obsidian,  pitchstone,  perlite,  pumice,  and  certain  quartzi- 

N 


1/8  Descriptive  Petrology. 

ferous  trachytic  lavas,  under  the  terms  rhyolite  and  liparite.1 
The  student  should  therefore  bear  in  mind  the  fact  that  the 
separation  of  the  vitreous  from  the  crystalline  rocks  refers 
merely  to  physical  differences  which  the  members  of  these 
two  sub-classes  respectively  present,  and  does  not  imply  any 
special  difference  in  their  chemical  composition.  These 
physical  differences  depend  upon  the  conditions  under 
which  solidification  was  effected,  whether  gradual  or  rapid. 
In  the  former  case  the  molten  mass  would  develop  crystals, 
in  the  latter  it  would  remain  amorphous :  it  would,  in  fact, 
result  in  a  more  or  less  perfect  glass.  In  these  natural 
glasses  it  is,  however,  common  to  find  crystallites  and 
crystals,  the  former  usually  developed  very  completely,  the 
latter  less  perfectly  formed  as  a  rule,  since  they  generally' 
present  rounded  boundaries,  or  their  angles,  if  any  exist, 
also  appear  rounded.  The  cause  of  these  rounded  boun- 
daries does  not,  as  yet,  seem  to  be  satisfactorily  deter- 
mined. It  is  known  that  fragments  of  rock  and  individual 
crystals  become  rounded  by  constant  attrition  during  their 
ejection  from,  and  their  returning  fall  into,  the  throat  of  a 
volcano  ;  and  since,  in  rather  rare  instances,  the  microscope 
shows  that  some  of  these  vitreous  lavas  contain  not  merely 
rounded  crystals,  but  also  well  rounded  fragments  of  other 
vitreous  rocks  of  a  quite  distinct  and  different  character 
from  that  of  the  matrix  in  which  they  are  enveloped,  it 
seems  possible  that  in  such  cases  the  rounded  crystals  and 
fragments  of  rock  represent  ejectamenta,  which,  rounded  by 
attrition,  and  lying  within  or  around  the  crater,  have  been 
taken  up  by  the  viscous  mass  of  lava  as  it  welled  over  them. 
If  this  were  the  case,  we  might  at  first  be  tempted  to  think 
that  the  rounding  was  due  to  the  superficial  fusion  of  the 

1  The  name  rhyolite,  from  £uo|  (a  lava  stream)  and  \idos,  was  in- 
troduced by  v.  Richthofen  in  1860,  and  included  certain  Hungarian 
quartz-trachytes,  which  showed  strong  evidence  of  viscous  fluxion,  and 
the  highly  silicated  vitreous  rocks  just  mentioned.  A  year  later  Justus 
Roth  applied  the  term  liparite  to  similar  crystalline  and  vitreous  rocks 
occurring  in  the  Lipari  Islands. 


Vitreous  Rocks.  179 

fragments  or  crystals,  just  as  fragments  of  minerals  become 
fused  in  a  borax  bead  before  the  blowpipe ;  and  it  may  be 
that  such  a  supposition  is  not  wholly  incorrect,  since,  although 
in  the  borax  bead  the  minute  fragment  as  it  fuses  becomes 
surrounded  by  visible  tortuously-twirling  strings  of  its  own 
molten  substance,  before  these  fused  products  become  per- 
fectly incorporated  with  the  borax  glass;  still  we  must 
remember  that  this  fused,  ropy  matter  is  visible,  because  it 
differs  in  density  from  the  fused  borax,  while  in  the  case  of 
vitreous  rock  fragments,  felspar  crystals,  &c.,  fusing  in  a 
highly  heated  vitreous  magma,  the  respective  specific  gravi- 
ties do  not  differ  sufficiently  to  render  the  phenomenon  of 
imperfect  incorporation  apparent. 

The  sp.  gr.  of  borax  is  171 

„  obsidian       „  2-4:1  —  2-57 

„  perlite          „  2-25 

„  sanidine       „  2*56  — 2*6 

„  plagioclase  „  2-56-276 

It  should,  however,  be  borne  in  mind  that  the  substance 
of  felspars,  which  are  the  principal  rounded  crystals  in 
vitreous  rocks,  is  approximately  colourless,  so  that  in  a 
colourless  magma  the  phenomena  of  imperfect  mixture  would 
not  be  apparent.  Such  phenomena  are,  however,  distinctly 
visible  in  some  obsidians  and  pitchstones,  in  which,  under 
the  microscope,  tortuous  lines  of  glass,  differing  markedly  in 
colour  or  tint  from  the  glass  in  which  they  lie  (fig.  75), 
denote,  no  doubt,  a  difference  in  the  relative  specific 
gravities  of  the  two  glasses.1  Such  included  glass  lines  and 
bands  in  hyaline  rhyolites,  although  they  show  us  that  the 
glass  is  not  homogeneous,  do  not  furnish  us  with  any  clue  as 
to  the  source  of  the  material  which  differs  from  its  matrix. 
It  cannot  well  be  imagined  that  the  rounded  crystals  and 

1  This  may  be  seen  in  the  obsidians  from  Tolcsva  in  Hungary, 
Truckee  Ferry  in  Nevada,  and  other  similar  rocks.  Kindred  phenomena 
may  be  seen  on  mixing  liquids  of  different  specific  gravities. 

N  2 


1 80  Descriptive  Petrology. 

fragments  in  these  vitreous  lavas  were  showered  down  on 
the  surface  of  the  viscous  lava  stream,  since  that  would 
imply  a  synchronous  eruption  of  lava  and  ejection  of  ashes, 
dust,  sand,  &c.,  from  the  same  vent,  for,  where  two  craters  are 
situated  near  one  another,  one  is  generally  at  rest  while  the 
other  is  active.  If  neither  of  the  foregoing  hypotheses  be 
adopted  there  seemingly  remains  but  one  other,  namely,  that 
the  crystals  have  been  developed  during  the  solidification  of 
the  rock,  and  that  the  rounded  contours  which  their  sections 
present  are  due  to  aborted  crystallisation,  such  as  that 
pointed  out  by  Poussin  and  Re'nard  as  occurring  in  the 
orthoclase  of  the  porphyrite  of  Mairus  in  Belgium.1  We 
may,  perhaps,  admit  with  safety  that  in  many  instances  the 
crystals  have  been  developed  in  the  rock ;  but,  if  we  admit 
it  in  all  cases,  how  are  we  to  account  for  the  included 
fragments  of  rock  which  may  occasionally  be  noticed  in 
microscopic  sections  of  these  lavas?  It  is  also  worthy  of 
note  that  the  same  section  may  exhibit  crystals  with  well- 
developed  angles  and  also  rounded  crystals  of  the  same 
mineral. 

Certain  structural  peculiarities  and  inclosures,  many  of 
which  can  only  be  observed  microscopically,  are  character- 
istic of  the  vitreous  rocks.  These  structures  or  inclosures 
do  not  always  individually  characterise  these  rocks,  since  it 
is  not  uncommon  to  find  crystals,  crystallites,  microliths, 
spherulites,  &c.,  all  developed  in  the  same  specimen. 

The  following  is  a  descriptive  list  of  the  principal  struc- 
tures which  occur  in  vitreous  rocks. 

Homogeneous. — This  condition  is  more  hypothetical  than 
real,  since,  when  examined  microscopically,  scarcely  any  of 
the  most  homogeneous-looking  obsidians  are  seen  to  be  free 
from  inclosures  of  microliths.  If,  however,  these  microliths 
and  other  inclosures  be  put  out  of  the  question,  the  glassy 

1  Memoire  sur  les  caracttres  mineral ogiques  et  stratigraphiques  des 
Roches  dites  Plutoniennes  de  la  Belgique  et  de  P  Ardenne  Fran$aise.  De 
la  Vallee  Poussin  et  Renard.  Bruxelles,  1876. 


Banded  and  Damascened  Structures. 


181 


.Fie.  74  A. 


matrix  in  which  they  lie  may  be  regarded  as  homogeneous, 
or  as  approximately  homogeneous,  although,  under  high 
powers,  it  often  shows  included  dusty  matter,  which  might 
exhibit  some  definite  characters  if  still  higher  powers  were 
employed.  It  may,  however,  be  stated  that,  as  a  rule, 
all  of  these  natural  glasses  contain  fine  dust  and  micro- 
liths. 

Banded. — This  structure  is  rendered  evident  by  the  inter- 
lamination  of  glasses  which  differ  in  tint,  or  by  the  segrega- 
tion of  granular  mat- 
ter in  strings  or  layers. 
The  bands  are  seldom 
continuous  for  any 
distance,  and  usually 
exist  merely  as  elon- 
gated lenticular 
streaks.  Fig.  74  A 
shows  the  banded 
appearance  of  a  sec- 
tion of  black  obsidian 
from  the  Island  of 
Ascension,  magnified 
50  diameters. 

Da  mascened. — The 
author  suggests  this 
term  as  a  convenient  one  by  which  to  describe  the  struc- 
ture shown  in  some  obsidians,  in  which  streaks  or  threads 
of  glass  are  contorted  in  a  confused  manner,  which  some- 
what resembles  the  markings  on  Damascus  sword-blades  or 
the  damascening  on  gun-barrels.  Fig.  75  represents  part  of 
a  section  of  a  red  obsidian,  from  Tolcsva,  in  Hungary,  mag- 
nified 50  diameters,  in  which  the  damascene  structure  is  well 
shown.  These  twisted  threads  of  glass  are  of  a  different  tint 
or  colour  to  that  of  the  glass  in  which  they  lie.  The 
appearance  which  they  present  when  seen  in  thin  sec- 
tion under  the  microscope  suggests  that  which  two  liquids 


182 


Descriptive  Petrology, 


of  different  density  exhibit  when  they  are  imperfectly  mixed 
and  slightly  agitated,  as  pointed  out  on  page  179. 

FIG.  75.  Perlitic. — A  struc- 

ture   especially   cha- 
racteristic     of      the 
rocks  termed  peril tes, 
but  sometimes  deve- 
loped      in        other 
vitreous   rocks    such 
as     trachylyte,     &c. 
This     structure    lias 
been  described  as  a 
phenomenon  attend- 
ant   upon     contrac- 
tion,   first    by    Pro- 
fessor   Bonney    and 
subsequently  by  the 
author,  who  was  at  the  time  ignorant  of  Professor  Bonney's 
conclusions.     These  conclusions  have  since  been  admirably 
demonstrated  by  Mr.  Allport's  examination  of  some  ancient 
perlites    occurring     in     Shropshire. 
The  structure  consists  in  the  deve- 
lopment of  numerous  minute  cracks 
which  exhibit  varying  curvature,  and 
produce   somewhat    concentric  and 
approximately  spheroidal  or  elliptical 
figures,   but  the  lines  which  bound 
these   forms   do  not  coalesce  as   a 
rule,  so  that  the   structure  may  be 
described  as  an  imperfect,  concen- 
tric,  shaly  one,   which,    on  a  large 
scale,     finds     a     parallel     in     the 
spheroidal   structure    developed    in 
some    basalts.      The    spheroids   in 
perlite  are  almost  invariably  found  to  lie  packed  between 
minute   rectilinear  fissures  which  traverse   the  rock  in  all 


FIG.  76. 


Spherulitic  Structure. 


183 


directions,  but  which  are  seldom  seen  to  cut  through  the 
spheroids.  Fig.  76  represents  a  section  of  perlite  from 
Buschbad,  near  Meissen,  Saxony,  magnified  about  10  diame- 
ters. The  latter  are,  however,  often  seen  to  be  traversed 
by  more  or  less  parallel  streams  of  microliths  which  bear  no 
relation,  or  observe  no  relative  disposition,  to  the  spheroids, 
thus  showing  that  the  perlitic  structure  had  no  existence 
when  the  rock  was  in  a  state  of  fluxion,  but  was  developed 
on  the  solidification  of  the  rock. 

Spherulitic. — This  is  a  structure  totally  distinct  from  that 
just  described  and  may  be  regarded  as  concretionary,  or  as 
resulting  from  incipient  crystallisation  around  certain  points 
or  nuclei.  The  nuclei,  when  they  exist,  consist  either  of  a 
granule  or  a  minute  crystal  or  crystallite,  but  most  commonly 
no  nucleus  is  discernible.  Spherulitic  structure  in  its  most 
rudimentary  form  seems  to  consist  in  the  segregation,  in 
spots,  of  glassy  matter,  of  a  different  colour  to  that  which 
constitutes  the  matrix,  and  often  contains  a  considerable 
quantity  of  very  fine  dust,  the 
nature  of  which  has  not  been 
ascertained  but  which  is  pro- 
bably magnetite.  The  glass 
constituting  the  spherulites  is 
usually  of  a  deep  yellowish- 
brown  colour  and,  in  very 
perfectly  developed  spherules, 
generally  forms  a  broad  zone, 
within  which  lies  clear  light- 
coloured  or  colourless  glassy 
matter  having  a  radiate  struc- 
ture, due  to  imperfect  crystalli- 
sation, while,  at  the  central  spot, 
from  which  the  crystals,  rods,  or 
fibres  emanate,  a  few  doubly 
refracting  granules  may  sometimes  be  observed.  In  some 
instances,  as  in  the  obsidians  of  the  Lipari  Islands,  a  per- 


FIG.  77- 


1 84 


Descriptive  Petrology. 


fectly  clear,  colourless,  but  very  narrow  outer  zone  surrounds 
the  brown  glassy  envelope  of  the  spherulite,  as  in  fig.  77. 
(Magnified  150  to  200  diameters).  In  vitreous  rocks  from 
the  last  named  locality,  in  those  from  the  Island  of  Ascen- 
FIG.  78.  sion,  and  in  many  other  ex- 

amples, the  spherulites  occur 
in  definite  layers  or  belts,  and. 
have,  in  many  cases,  been 
elongated  in  the  direction  in 
which  the  lava- stream  flowed ; 
at  times  they  even  coalesce 
and  form  more  or  less  continu- 
ous bands,  as  in  fig.  78  (mag- 
nified 22  diameters),  which 
represents  part  of  a  band  of 
coalesced  spherulites  in  the 
obsidian  of  Rocche  Rosse,  Lipari.  Occasionally,  but  very 
rarely,  spherulitic  structure  is  so  extensively  developed  in 
vitreous  rocks  that  the  whole  mass  consists  of  closely  packed 
spherulites,  between  which  only  small  patches  of  the  glassy 
matrix  can  here  and  there  be  discerned,  while  the  spherulites 
are  so  closely  crowded  together  that  their  boundaries  are  no 
longer  spherical,  but,  by  compression,  assume  polygonal 
forms.  Spherulitic  structure  is  sometimes  developed  in 
artificial  glass.  A  fragment  of  a  plate-glass  window,  from 
a  house  which  had  been  burnt  down,  exhibited  colonies  of 
spherulites,  when  examined  under  the  microscope. 

Axiolitic. — A  structure  is  developed  in  some  of  the 
vitreous  rocks  of  Nevada,  U.S.,  and  elsewhere,  the  indivi- 
dual components  of  which  have  been  termed  axiolites  by 
Zirkel.1  These  appear  to  be  somewhat  analogous  in 
structure  to  spherulites  ;  elongated  lenticular  and  curved 
zones  of  brownish  glass  forming  the  envelope  of  a  smaller 
corresponding  mass  of  paler  vitreous  matter,  in  which 

1  Microscopic  Petrography,  Zirkel,  U.  S.  Exploration  of  the  Fortieth 
Parallel. 


Devitrification. 


FIG.  79. 


incipient  crystallisation  or  fibrous  structure  trends  at  right 
angles  to  the  inner  surfaces  of  the  envelope  towards  a 
longitudinal  median  line.  The  great  diversity  exhibited  by 
such  structures  is  well  shown  in  the  work  cited  in  the  foot- 
note from  which  the  accompanying  figure  (79)  is  copied.  The 
figure  represents  the  axiolitic 
structure  visible  in  a  rhyolite 
from  N.W.  of  Wadsworth, 
Nevada,  U.S. 

Porphyritic. — This  term  is 
applied  to  vitreous,  just  as  to 
other    rocks,    implying    that 
isolated     crystals     distinctly 
visible    to    the    naked    eye 
occur  in  them.     The   appli- 
cation   of    this  term  has   in    — — " 
all    cases    a    purely    arbitrary  limit, 
refers  to 
to  their 
occur 


a  purely  arbitrary  limit,  since  it  not  merely 
the  mode  of  occurrence  of  the  crystals,  but  also 
size  ;  rocks  in  which  very  small  isolated  crystals 
only  being  spoken  of  as  micro-porphyritic,  simply 
because,  from  their  small  dimensions,  they  do  not  convey  to 
the  naked  eye  the  blotched  appearance  which  characterises 
the  commonly  recognised  porphyries. 

Ttichitic  and  Microlitic  are  terms  which  might  also  be 
given  to  those  vitreous  rocks  which  contain  multitudes  of 
the  bodies  already  described  as  trichrites  and  microliths  ; 
but  as  nearly  all  vitreous  rocks  are  more  or  less  microlitic, 
and  as  the  word  'trichitic'  sounds  inconveniently  like  the 
adjective  '  trachytic,'  which  latter  is  often  applied  to  rocks 
of  this  class,  such  terms  as  trichitic  and  microlitic  are 
perhaps  better  left  alone. 

Devitrified. — This  implies  that  the  rock  has  undergone, 
to  a  greater  or  less  extent,  certain  physical  changes  which 
cause  it  no  longer  to  behave  as  a  glass,  its  vitreous  character 
being  partially  or  completely  destroyed  by  the  development 
either  of  microliths,  crystalline  granules,  or  crystals.  The 


1 86  Descriptive  Petrology. 

ultimate  stage  of  crystalline-granular  devitrification  is  felsitic 
matter,  and,  when  a  rock  has  undergone  complete  change  of 
this  kind,  it  is  only  possible  to  arrive  at  conclusions  as  to  its 
once  vitreous  nature,  by  means  of  those  structural  peculiar- 
ities which  indicate  former  fluxion,  and,  should  those 
characters  fail  to  be  very  well  marked,  it  is,  as  a  rule,  most 
hazardous  to  jump  at  any  conclusions  concerning  the 
original  condition  of  the  rock. 

Filiform. — A  condition  occasionally,  but  rarely,  met  with  ; 
as  in  the  filiform  lava  of  Hawaii,  in  the  Sandwich  Islands, 
known  as  Pe'le's  hair,  in  which  molten  vitreous  lava  has 
been  frayed  out  and  blown  by  the  wind  into  long  and 
extremely  slender  glassy  threads,  which  commonly  terminate 
in  little  fused  knobs  or  pellets.  This  structure  is  also 
produced  artificially  in  blast-furnace  slags. 

OBSIDIAN. 

C  Obsidian  results  from  the  quick  solidification  of  lavas 
which,  if  slowly  cooled,  would  develope  crystalline  structure 
and  assume  the  character  of  trachyte,  Hparite,  &c.,  rocks 
which  contain  over  60  per  cent,  of  silica.  In  obsidian  no 
crystalline  structure  is  developed ;  it  is  a  true,  natural  glassT) 
Nevertheless,  obsidians  frequently  contain  microliths;  ana, 
when  spherulitic^  the  spherulites  commonly  show  a  radial 
crystalline  or  fibrous  structure.  Obsidians  present  a  very 
homogeneous  appearance  and  a  strong  vitreous  lustre.  Their 
fracture  is  eminently  conchoidal.  (  They  vary  somewhat  in 
colour,  but  are  mostly  black  or  grey.  In  thin  splinters  they 
are  all  more  or  less  transparent, 

=NObsidians  also  vary  in  chemical  composition^  The  silica 
may  be  estimated  at  from  60  to  80  per  cent.)  the  alumina  at 
18  to  19  per  cent.,  while  the  remaining  constituents  are 
potash  or  soda,  lime,  magnesia,  peroxide  of  iron,  and  occa- 
sionally as  much  as  0-5  of  water.  Their  specific  gravity 
ranges  between  2^4  and  2*5,  and  the  hardness  equals  6  to  7. 


Obsidian.  187 

(Those  which  contain  the  largest  proportion  of  silica  are  only 
slightly  acted  upon  by  acids^ 

(Before  the  blowpipe  obsidian  is  fusible  on  the  edges  of 
thin  splinters^  Sections  of  obsidian  when  placed  under  the 
microscope  between  crossed  Nicols  exhibit  no  double  re- 
fraction, the  field  appearing  quite  dark ;  but  this  dark  field 
is  usually  thickly  studded  with  bright  doubly-refracting 
microliths,  and  under  moderately  high  powers  crystallites  of 
varied  forms,  exhibiting  structural  peculiarities  of  excessive 
beauty  and  interest,  may  often  be  met  with  in  great  profusion. 
(There  are,  however,  some  obsidians,  such  as  the  pseudo- 
chrysolite  or  bouteillen stein,  which  occurs  as  rounded  pebbles 
in  sand]  at  Moldauthein  in  Bohemia,  and  in  some  of  the  tuffs 
near  Mont  Dore  in  Auvergne,  which  show  no  crystallites 
under  the  microscope,  and  equally  pure  obsidians  occur  in 
one  or  two  localities  in  New  Zealand  and  in  Iceland.  All 
of  these,  however,  contain  great  numbers  of  gas  pores. 

The  crystallites  which  occur  in  obsidian  vary  so  greatly 
in  form  that  mere  descriptions  of  them  would  be  of  little 
use  to  the  student.  The  precise  mineral  species  which  they 
represent  are  in  many  cases  undetermined,  but  it  is  probable 
that  many  of  them  are  incipient  felspar  crystals.  In  some 
of  the  small  crystals  or  microliths,  which  are  so  common  in 
rocks  of  this  class,  it  is  interesting  to  note  the  gradual  deve- 
lopment of  structure  which  may  sometimes  be  seen  in  a 
single  microscopic  section ;  in  one  place  simply  a  comb-like 
crystalline  growth  springing  from  minute  tapering  rods,  which 
constitute,  as  it  were,  the  visible  axes  of  these  little  crystal- 
lites ;  in  another,  a  microlith,  or  small  crystal,  in  which  may 
be  seen  a  structure  identical  with  the  preceding,  and  which 
seems  to  show  the  plan  upon  which  it  has  been  built  up,  to 
be  in  fact  the  framework  upon  which  it  has  been  developed. 
The  little  axial  rods,  if  they  may  be  so  termed,  are  not 
always  straight ;  at  times  they  have  a  somewhat  sigmoidal 
flexure,  at  others  they  occur  in  pairs  arranged  like  two  bows 
set  back  to  back.  This  disposition  is  occasionally  coupled 


1 88  Descriptive  Petrology. 

with  the  intermediate  development  of  a  small,  square,  rect- 
angular, or  rhomboidal  mass,  from  the  four  corners  of  which 
the  apparent  homologues  of  these  arcuate  rods  sprout  out 
like  horns,  as  in  fig.  80,  while  the  whole  is  surrounded  by  a 
FIG.  80.  hyaline  border,  whose  external  boundary  and 
occasionally  striated  structure  indicate  differen- 
tiation of  the  surrounding  glassy  magma  and 
the  incipient  extension  •  of  crystalline  develop- 
ment. Similar  borders  often  surround  crystal- 
lites which  give  sections  like  those  which  an 
^350™'  *  elongated  pyramid  would  afford ;  and  cruciform 
groupings,  which  closely  resemble  aggregates  of  such 
pyramids,  may  also  be  seen  at  times  in  obsidians. 

(Besides  the  crystallites  just  mentioned,  it  is  common  to 
find  spherulites  developed  in  these  lavas.)  In  their  most 
rudimentary  condition  they  occasionally  seem  to  be  repre- 
sented by  blotches  of  a  glass,  of  deeper  colour  than  that  of 
the  surrounding  matrix.  Generally  the  most  perfectly  de- 
veloped spherulites  have  a  somewhat  broad  border  of  glass, 
which  appears  reddish-brown  by  transmitted  light,  and  sur- 
rounds a  central  spherule  of  clear  and  often  almost  colour- 
less glass,  in  which  a  radiate  structure  is  developed,  while, 
in  some  instances,  the  whole  spherulite  is  surrounded  by  a 
narrow  colourless  envelope  of  clear  glass.  These  spherulites 
are  sometimes  elongated  in  the  direction  in  which  the  once 
viscous  stream  of  obsidian  flowed,  and  this  elongation  has 
occasionally  taken  place  to  such  an  extent  that  the  spheru- 
lites have  coalesced,  and  formed  more  or  less  continuous 
bands,  of  which  the  central  portion  consists  of  vitreous  and, 
at  times,  micro-crystalline  matter.  This  is  cased  in  an  outer 
envelope  of  glassy  matter  which  appears  reddish-brown  by 
transmitted  light,  and  generally  snow-white  or  greyish  by 
reflected  illumination.  Occasionally  this  is  surrounded  by  a 
thin  external  coat  of  clear,  colourless  glass,  which,  unlike 
the  clear  absorption  areas  seen  around  the  crystallites  and 
dust  segregations  in  some  vitreous  rocks,  is  bounded  by  a 


Obsidian. 


189 


FIG.  81. 


sharp  line  of  demarcation  from  the  glass  which  constitutes 

the  matrix.     Such  spherulitic  bands  have  sometimes,  when 

the  coalescence  of  the  spherulites  has  only  extended  to  their 

cortical  zones,  merely  the  aspect  of  beads  closely  strung 

together  ;  but,  in  cases  where  the  coalescence  has  been  more 

complete,  the  boundaries   of  the  bands  are  approximately 

parallel  straight  lines,  so  that  the  structure  of  such  a  band 

or  string  may  be  diagram- 

matically   represented    as 

in    fig.   8  1,   a  being   the 

transverse  and  b  the  longi- 

tudinal section.    It  is  not, 

however,  to  be  supposed 

that,  where    vast    multi- 

tudes  of   spherulites   are  developed   on   the   same   plane, 

transverse  sections  such  as  a  (fig.  81)  are  invariably  to  be 

procured,  and  in    such    cases  we    may    assume    that    the 

coalescence  of  the  spherulites  gives  rise  to  sheets,  rather 

than  strings,  the  vertical  sections  through  such  sheets  afford- 

ing  in  all   directions   a  disposition   corresponding  with  b 

(fig.  81).     In  some  cases,  as  in  the  spherulitic  obsidian  of 

Rocche  Rosse  in  the 

Island     of      Lipari, 

clear  colourless  rods 

of  glass  are  seen  to 

have  been  extruded 

through  the  cortical 

layers  of  the  spheru- 

litic bands  into  the 

surrounding     glassy 

matrix      (fig.       82), 

which     are     further 

enlargements  of  the 

little  rods  shown  in 

fig.  78,  page  184. 

After  emergence  from  the  band  they  are  frequently  hooked 


FIG.  82. 


190 


Descriptive  Petrology. 


FIG.  83. 


or  bent,  but  not,  as  a  rule,  in  any  mutually  definite  direction, 
and  in  most  sections  they  are  seen  either  to  terminate 
blindly  in  rounded  ends,  or  to  be  cut  off  on  the  ground 
surfaces  of  the  preparation. 

( Small  crystals  and  microliths,  as  already  stated,  are  of 
common  occurrence  in  some  obsidians.  In  many  cases 
they  can  be  safely  identified  with  recognised  minerals^  such 
as  sanidine,  plagioclase,  augite,  hornblende,  olivine,  tourma- 
line, zircon,  magnesian  mica,  specular  iron,  and  magnetite. 

The  felspars  occur  in  small  prisms.  The  augite  and 
hornblende  exist  either  as  distinct  crystals,  similar  to  the 
ordinary  forms,  or  as  minute  acicular  bodies  and  spicular 
forms  ('  belonites  ')  which  are  often  bordered  by  imperfectly 
radiate,  fibrous  or  hazy,  and  almost  dendritic  tufts,  which 
cause  them  somewhat  to  resemble  the  fronds  of  ferns. 
Beautiful  crystallites  of  this  description  may  be  seen  in  some 
of  the  pitchstones  of  Arran  (fig.  83),  and 
have  been  identified  by  S.  Allport  as 
augite.  The  crystals  of  olivine  are  always 
of  moderate  size  and  no  microliths  of 
this  mineral  have  as  yet  been  detected. 
The  occurrence  of  tourmaline  and  zircon 
has  not  been  definitely  determined,  but 
certain  prisms  belonging  to  the  hexagonal 
or  rhombohedral  system  have  been  thought 
to  be  tourmaline,  while  some  tetragonal 
forms  are  regarded  as  zircon. 
The  magnetite  occurs  in  opaque  octahedral  crystals  or 
granules,  and  the  specular  iron  in  little  yellowish-red  or 
orange-coloured  hexagonal  tabular  crystals.  The  magnesian 
mica  forms  small,  deep,  reddish-brown  scales  and  crystals 
which  resemble  those  of  specular  iron,  but  although  they 
show  no  dichroism  when  their  basal  planes  coincide  with 
the  planes  of  section,  yet  their  sections  are  very  strongly 
dichroic  when  cut  transversely  or  obliquely  to  the  basis. 
Obsidian  occurs  on  a  large  scale  as  lava-flows,  which 


Pumice. 


191 


frequently  present  very  rough  and  jagged  surfaces.  That 
forming  the  Rocche  Rosse  in  the  island  of  Lipari  is  a  good 
example.  The  accompanying  figure  of  this  obsidian  stream, 
which  has  issued  from  the  crater  of  Campo  Bianco,  breach- 
ing one  side  of  the  crater,  and  flowing  over  the  white  pumice 
tuffs,  of  which  the  cone  is  composed,  is  copied  by  permission 
of  Prof.  J.  W.  Judd  from  one  of  the  sketches  published 
in  his  '  Contributions  to  the  Study  of  Volcanoes,'  Geological 
Magazine,  Decade  II.  Vol.  ii.  No.  2,  p.  66. 

FIG.  83  A. 


fobsidian  at  times  becomes  vesicular./'  In  some  of  the 
obsidians  of  Hawaii  the  vesicles  are  quite'spherical ;  in  others 
they  are  elongated  or  otherwise  distorted.  Occasionally  these 
rocks  are  very  finely  vesicular,  as  is  the  case  with  some  of  the 
obsidians  of  the  Lipari  Isles,  and^the  vesicles  are  at  times  so 
numerous  that  the  rock  acquires  quite  a  frothy  character, 
and  passes  into  pumice./) 
\Obsidians  occur  in  districts  where  trachytic  rocks  are 


common. 


PUMICE. 

(Pumice  is  a  porous,  vesicular  glass ;  the  vesicles  being 
frequently  elongated)  sometimes  in  a  more  or  less  definite 


192  Descriptive  Petrology. 

direction,  while  at  others  they  anastomose,  and  give  rise  to 
an  irregular  network  of  fibrous,  intervesicular  matter. 

uPumice  varies  considerably  in  chemical  composition, 
the  percentage  of  silica  ranging  between  57  and  73.^}  The 
alumina  varies  from  9  to  20  per  cent.,  and  the  remainder 
consists  of  lime,  magnesia,  potash,  soda,  and  peroxide  of 
iron.  Water  is  also  present.  The  specific  gravity  of  pumice 
varies  from  1*9  to  2*5. 

/The  fusibility  before  the  blowpipe  is  greater  in  some 
specimens  than  in  others) 

("When  examined  microscopically  some  pumice  appears 
to  consist  of  interlacing  or  anastomosing  vitreous  fibresXew 
or  no  microliths  or  crystallites  being  developed  in  the  glassy 
matter.  In  others  microliths  and  small  crystals  occur  in 
abundance,  and  frequently  show  the  stream-like  disposition 
which  results  from  fluxion,  or  from  the  drawing  out  of  small 
portions  of  viscous  lava.  The  microliths,  when  they  can 
be  determined,  are  found  in  most  instances  to  be  felspars, 
both  monoclinic  and  triclinic.  Magnetite  is  also  of  common 
occurrence. 

(  Pumice  is  developed  on  the  surfaces  of  obsidian  streams, 
and  in  such  cases  can  only  be  regarded  as  a  highly  vesicular, 
spongy,  or  fibrous  condition  of  obsidian.  The  porphyritic 
development  of  felspar  crystals  in  pumice  begets  the  rock 
termed  trachyte-pumice.  Pumice  also  occurs  in  the  form 
of  loose  ejected  blocks  and  fragments.  These  ejectamenta 
sometimes  constitute  volcanic  cones.  ~\  In  fig.  83  A  the  cone, 
which  is  partially  broken  down  by  me  stream  of  obsidian, 
consists  of  pumice  fragments. 

PERLITE  (Pearlite,  Pearlstone). 

Perlite  is  sometimes ,  quite  glassy  in  appearance,  but  it 
more  frequently  exhibits  a  shimmering,  pearly,  enamel-like, 
or  greasy  aspect  oh  recently  fractured  surfaces.  The  colour 
is  mostly  pale-greyish,  bluish-grey,  or  yellowish-brown.  It 
often  appears  to  consist  in  great  part  of  spherical  or  round- 


Per  lite.  193 

ish  grains,  which  have  a  somewhat  concentric  shaly  structure. 
These,  although  commonly  individualised,  very  frequently 
coalesce,  the  rock,  in  such  cases,  assuming  a  more  homo- 
geneous character. 

In  chemical  composition  the  perlites  approximate  to 
the  quartz-trachytes  (the  rhyolites  proper,  or  the  liparites  of 
Von  Richthofen1).  They  are  highly  silicated,  containing 
from  70  to  over  80  per  cent,  of  silica.  When  heated  they 
give  off  water,  the  amount  varying  from  2  to  4  per  cent. 
Their  hardness  is  about  6. 

Perlite  must  be  regarded  as  the  vitreous  condition  of  the 
felsitic  rhyolites,  and,  like  other  vitreous  rocks,  plays  a  very 
subordinate  part  in  the  constitution  of  the  hitherto  explored 
portions  of  the  earth's  crust,  when  compared  with  those  rocks 
of  which  it  is  the  vitreous  representative.  It  should,  how- 
ever, be  borne  in  mind  that  the  originally  vitreous  character 
of  many  eruptive  rocks  has  yet  to  be  discovered,  since, 
owing  to  devitrification,  they  frequently  present  appearances 
which,  in  the  absence  of  microscopic  investigation,  afford  no 
clue  to  the  physical  characters  which  they  possessed  at  the 
time  of  their  eruption.  It  is  indeed  more  than  probable 
that  many  of  the  so-called  hornstones,  felstones,  and  even 
rocks,  which  were  mapped  by  the  older  geologists  as  green- 
stones, are  merely  rhyolitic  rocks  in  a  devitrified  condition. 
In  this  country  the  researches  of  Professors  Bonney  and 
Judd,  Mr.  Allport.  and  the  author,  have  already  demonstra- 
ted the  existence  of  rocks  of  a  rhyolitic  type,  of  which  the 
real  characters  had  previously  been  overlooked.  When 
examined  microscopically  thin  sections  of  perlite  exhibit 
numerous  fissures,  and  between  these  fissures  great  numbers 
of  somewhat  concentric  cracks  are  visible,  causing  a  sepa- 
ration of  the  rock  into  more  or  less  regular  spheroids.  The 
cracks  do  not  appear  to  join  up  in  continuous  ellipses,  but 

% 

1  The  term  liparite  has  been  applied  by  Roth  to  th%  whole  of  .the 
rhyolites.  Von  Richthofen,  however,  limits  the  use  of  the^rm  to  the 
felsitic  rhyolites,  or  rhyolites  proper. 

O 


IQ4  Descriptive  Petrology. 

thin  off  and  at  times  overlap.  They  nevertheless  form  ap- 
proximately concentric  envelopes  around  the  spheroidal 
nucleus  of  glass.  It  is  worthy  of  remark  that  these  bodies 
are  seldom  or  never  traversed  by  the  straight  cracks  which 
run  in  various  directions  through  the  rock,  but  lie  between 
them,  often  closely  packed,  distorted,  and  apparently  com- 
pressed against  the  planes  of  the  straight  fissures.  This 
indicates  that  the  straight  fissures  were  formed  first  and  that 
the  spheroidal  or  perlitic  structure,  as  it  may  be  termed, 
was  subsequently  developed. 

Streams  of  microliths  commonly  occur  in  these  rocks, 
and  they  traverse  the  perlitic  bodies  without  the  slightest 
indication  of  deflection.  They  have,  in  fact,  been  quite 
uninfluenced  in  their  direction  of  flow  by  the  minute  structural 
planes  and  elliptical  cracks  which  occur  so  plentifully  in  the 
rock. 

Crystals,  both  megascopic  and  microscopic,  occur  in 
considerable  numbers  in  some  perlites.  They  consist  prin- 
cipally of  sanidine  and  plagioclastic  felspars,  magnesian 
mica,  magnetite,  and  occasionally  specular  iron.  The  mi- 
croscopic inclosures  consist  mostly  of  felspar-microliths, 
trichites,  and  belonites.  Spherulitic  structures  are  also 
present  at  times. 

That  the  perlitic  structure  has  probably  resulted  from 
the  development  of  more  or  less  concentric  zones  of  con- 
traction on  cooling  has  been  pointed  out,  both  by  Professor 
Bonney,  and  by  the  author.  Perlitic  structure  bears  a  some- 
what close  relation  to  the  larger  spheroidal  structure  which 
is  occasionally  to  be  seen  in  basalt. 

Indications  of  perlitic  structure  may  be  observed  in  thin 
sections  of  vitreous  rocks  other  than  perlite,  and  the  author 
has  noted  the  incipient  development  of  this  structure  in  an 
Irish  tachylyte  (Journ.  Royal  Geological  Society,  Ireland, 
vol.  iv.,  p.  230),  thus  showing  that  the  structure,  although 
characteristic  of  some  highly  silicated  vitreous  rocks,  is  not 
exclusively  peculiar  to  them. 


l 


Pitchstone.  195 

Professor  A.  von  Lasaulx  points  out  the  fact [  that  many 
spherulitic  rocks  were  formerly  regarded  as  perlites,  but  he 
adds  that  spherulitic  and  perlitic  structures  are  totally  diffe- 
rent, since  the  latter  consist  merely  of  little  masses  of  glass, 
while  the  former  are  '  crystalline  individualisations.' 

Spherulites  may  in  fact  be  regarded  as  spots  of  devitrifi- 
cation, while  perlitic  structure  is  simply  a  phase  of  fission 
resulting  from  contraction  on  cooling,  the  glass  included  by 
the  elliptical  fissures  in  no  way  differing  from  the  surround- 
ing glass. 

Pitchstone.—t{\\&  pitchstones  may  be  regarded  as  vitre- 
ous conditions  of  trachyte  on  the  one  hand,  and,  on 
the  other,  of  those  rocks  which  range  from  granites  to 
felstones,  including  the  porphyritic  varieties  of  those  rocks) 
such  as  quartz-porphyry,  porphyritic  felstone,  &c.  It  is 
true  that  pitchstone  is  not  recognised  as  a  vitreous  condi- 
tion of  granite,  but  since  passages  are  known  from  granite 
into  quartz-porphyry,  and  from  quartz-porphyry  into  fel- 
stone, it  seems  that  we  may  fairly  be  allowed  to  regard 
the  vitreous  equivalent  of  the  felstones  and  quartz-porphyries 
as,  at  all  events,  an  indirect  vitreous  phase  of  granite,  which 
appears  more  directly  to  find  its  rhyolitic  representative  in 
the  nevadites.  The  pitchstones  are  classed  as  trachytic  and 
felsitic.  The  devitrification  of  the  trachytic  pitchstones  is 
effected  by  the  development  of  microliths,  which  for  the 
most  part  consist  of  sanidine  and  hornblende,  while  the 
felsitic  pitchstones  become  devi trifled  by  the  setting  up  of  a 
micro-felsitic  or,  as  v.  Lasaulx  terms  it,  a  micro-aphanitic 
structure. 

(The  pitchstones  have,  as  a  rule,  a  perfectly  conchoidal 

fracture,  sometimes   rather  splintery,   and  a  more   or  less 

greasy  semi- vitreous,  or  pitch-like  lustre")  whence  the  name. 

<^  They  are  mostly  blackish-green,  dark  olive  green,  or  brown. 

Occasionally  they  are  red  or  dull  yellow.) 

1  Elemente  der  Petrographie,  p.  223.    Bonn,  1875. 
O  2 


1 96  Descriptive  Petrology. 

vln  chemical  composition  they  resemble  obsidian,  but 
contain  from  four  to  ten  per  cent,  of  water/)  If,  however, 
the  water  be  omitted,  and  the  other  constituents  brought  up 
to  100,  the  composition  of  pitchstone  may  be  .regarded  as 
approximately  identical  with  that  of  trachyte.  The  silica 
ranges  from  63  to  73  per  cent.,  the  alumina  from  9  to 
13,  and  the  alkalies  from  2  to  8  per  cent.  Before  the 
blowpipe  some  varieties  do  not  fuse  so  readily  as  others, 
but  they  all  melt  either  to  a  frothy  glass  or  to  a  greenish  or 
greyish  enamel,  and  yield  water  when  heated  in  a  tube. 

The  pitchstones  have  a  hardness  of  5  to  6,  and  a  specific 
gravity  of  2  "2  to  2  "4.  (jThey  are  not  acted  upon  by  acids) 

The  pitchstones,  as  already  stated,  may  be  divided  into 
two  groups — the  trachytic  pitchstones  and  the  felsitic  pitch- 
stones.  The  former  are  related  both  geologically  and  in 
mineral  constitution  to  the  liparites,  while  the  latter  are  re- 
lated to  the  quartz-porphyries  and  felstones.  With  regard 
to  the  trachytic  pitchstones,  Rosenbusch  observes1  that  no 
sharp  line  of  demarcation  can  be  drawn  between  them  and 
the  glass-magma-liparites  which  contain  water. 

Sections  of  pitchstone,  when  examined  microscopically, 
appear,  like  other  glasses  and  amorphous  substances,  dark 
between  crossed  Nicols.  (By  ordinary  illumination  they 
appear  either  as  perfectly  homogeneous  glass  containing 
crystals  and  microliths,  or  they  exhibit  streaks  of  glass  of  a 
deeper  or  paler  tint  than  that  of  the  surrounding  matrix^} 
which  in  their  disposition  at  once  suggest  the  presence  of 
fluxion  structure.  In  the  latter  cases,  as  well  as  in  the 
former,  crystals  are  of  common  occurrence,  and  microliths 
are  usually  developed  in  abundance. 

The  crystals  are  mostly  sanidine,  hornblende,  and  mag- 
netite, but  plagioclase  is  not  uncommon. 

The  microliths  may  often  be  recognised  as  consisting  of 
the  above-mentioned   minerals,  and    in  some  pitchstones, 
especially  in  those  from  the   Isle   of  Arran,  microliths  of 
1  Mik.  Phys.  d.  Massigen  Gesteine,  p.  160, 


Pitchstone. 


197 


The  pitchstone  of  Corriegills  in 


FIG.  84 


augite  are  also  plentiful. 
Arran  shows  these 
augite  microliths  or 
belonites  in  great 
profusion,  forming 
more  or  less  stellate 
groups  of  pale  green- 
ish forms,  which 
somewhat  resemble 
the  fronds  of  ferns, 
and  which  have  been 
admirably  figured 
and  described  by 
Zirkel,  Allport,  Vo- 
gelsang, and  other 
microscopists.  Fig. 
84  represents  a  mag- 
nified section  of  Arran  pitchstone,  and  is  copied,  by  per- 
mission of  Prof.  Zirkel,  from  his  '  Mikroskopische  BescharTen- 
heit  der  Mineralien  u.  Gesteine.' 

The  glass  of  most  pitchstones  appears,  when  light  is  trans- 
mitted through  thin  sections,  of  a  pale  yellowish,  greenish,  or 
brownish  tint,  but  it  is  scarcely  safe  to  venture  a  decided 
opinion  upon  the  precise  nature  of  the  pigment.  Some- 
times, but  not  in  all  cases,  there  appears  to  have  been 
an  abstraction  of  this  pigment  from  those  portions  of  the 
glassy  magma  which  immediately  surround  the  microliths 
of  magnetite  and-  augite,  this  absorption  of  pigment  pro- 
ducing a  comparatively  clear,  colourless  ring  around  these 
bodies.  QSteam  pores,  and  occasionally  spherulitic  struc- 
tures, are  met  with  in  pitchstonesp  The  microliths  in  these 
rocks  usually  lie  in  streams,  which-  sweep  round  the  larger 
imbedded  crystals,  and  indicate,  in  a  marked'  manner,  the 
originally  viscid  condition  of  the  matrix  in  which  they  occur. 

The  felsitic  pitchstones  bear  much  the  same  relation 
to  the  felstones  that  the  trachytic  pitchstones  bear  to  the 


198  Descriptive  Petrology. 

trachytes,  while  the  porphyritic  varieties,  termed  pitchstone 
porphyry,  may  also  be  regarded  as  the  vitreous  equivalent  of 
quartz-porphyry.  To  these  pitchstone  porphyries  Vogelsang 
has  given  the  name  vitrophyre. 

(The  principal   porphyritic   crystals    in  these  rocks  are 
quartz,  sanidine,  plagioclase,  augite,  hornblende,  magnetite, 
and  biotite.     They  also  at  times  contain  inclosures  of  glass,) 
which  are  sometimes  devitrified  by  conversion  into  micro- 
felsitic  matter,  at  other  times,  by  the  development  of  microliths. 

The  devitrification  of  the  magma  of  the  felsitic  pitch- 
stones  is  as  a  rule  micro-felsitic,  while  that  of  the  trachytic 
pitchstone  is  effected  by  the  development  of  microliths. 
Microliths,  some  doubly  refracting  and  some  singly  refract- 
ing, occur  however  in  the  .glasses  of  nearly,  if  not  all  diffe- 
rent varieties  of  pitchstone.  Inclosures  of  fluid,  containing 
mobile  bubbles,  are  of  rare  occurrence  in  these  rocks. 

The  devitrification  of  the  porphyritic  and  felsitic  pitch- 
stones  does  not,  as  a  rule,  take  place  uniformly,  but  occurs 
in  a  seemingly  capricious  manner,  often  being  developed  in 
irregularly- distributed  and  irregularly-shaped  patches,  some- 
times occurring  in  spots,  sometimes  in  strings,  which  usually 
indicate  fluxion-structure.  Dark  granules  also  occur  in 
these  rocks,  which  possibly  represent  magnetite,  and  these 
granules  frequently  assume  a  string-like  or  banded  arrange- 
ment, while  irregular  strings  or  lenticular  flecks  of  included 
glass  also  denote  a  former  state  of  fluxion. 

In  their  most  perfectly  vitreous  conditions  it  seems  that 
no  sharp  line  of  demarcation  can  be  drawn  between  the 
different  varieties  of  pitchstone  here  described,  for  although 
differences  may,  and  doubtless  do  exist,  still  our  power  of 
appreciating  those  differences  is  limited  or  nil,  so  long  as 
devitrification  has  not  supervened,  and  so  indicated,  by  inci- 
pient crystalline  development,  the  true  petrological  affinities 
of  the  glass.  The  crystals  which  occur  porphyritically  in 
the  different  varieties  afford  us  a  very  imperfect  clue  to  these 
relations,  simply  because  they  represent  for  the  most  part 


Tachylyte.  '  /  ,      199 

-"  \"  A 

identical  mineral  species.     The  characters  of  A\JQ  rocks  ivdth 

which  they  are  associated,  and  of  which  they  re£fe/s,ent  the  /) 
vitreous  conditions,  give  us  however  a  more /exact  ncr&cjn,  pf 
the  places  which  they  ought  to  occupy  in  our  ossification)  • 
of  them.  '/,-»  v  fc 

Indeed  it  may  not  be  indiscreet  to  believe  that  in  rnar^, 
if  not  in  all  instances,  the  crystalline  equivalents  of  these  . 
vitreous  rocks  do  but  represent  an  advanced  phase  of 
devitrification,  and  that  all  trachytes  were  once  trachytic 
pitch  stones,  and  that  all  felstones  were  once  felsitic  pitch- 
stones,  either  at  the  time  of  or  prior  to  their  eruption. 

Tachylyte. — The  rocks  included  under  this  name  must 
be  regarded  as  vitreous  conditions  of  basic  rocks,  especially 
of  basalts.  These  glassy  basalts  are  termed  basaltvitro- 
phyres  by  Rosenbusch,  and  he  subdivides  them  into  tachy- 
lytes,  or  those  which  are  soluble  in  acids,  and  hyalomelanes 
or  those  which  are  insoluble  in  acids.  The  tachylytes  occur 
mostly  as  salbands,  or  thin  crusts  at  the  sides  or  margins  of 
basalt  dykes,  but  the  essentially  vitreous  basic  lavas,  such  as 
those  of  Kilauea  in  the  Sandwich  Islands,  which  form  actual 
flows  of  considerable  magnitude,  constitute,  as  pointed  out 
by  Cohen,  independent  rock  masses.  These  Kilauea  lavas 
are,  as  a  rule,  rich  in  olivine,  and  are  for  the  most  part 
highly  vesicular.  Tachylyte  also  occurs  lining  or  filling 
vesicles  or  cavities  in  basalt.  The  tachylytes  are  black  or 
brown  glasses  and  somewhat  resemble  obsidian,  but  when 
struck  with  the  hammer  they  do  not  usually  afford  large 
flakes  and  extensive  conchoidal  fractures  like  obsidian,  but 
generally  break  up  into  small  irregular  fragments  and  splin- 
ters. They  also  differ  from  obsidian  in  point  of  fusibility 
and  chemical  composition,  tachylyte  only  containing  from 
5°  to  55  Per  cent,  of  silica.  The  remaining  constituents  are 
alumina,  protoxide  of  iron,  sometimes  peroxide  of  iron,  lime, 
magnesia,  potash,  soda,  and  usually  about  6  or  7  per  cent, 
of  water.  From  the  few  analyses  of  tachylyte  which  have 
hitherto  been  made,  the  composition  appears  to  be  rather 


200 


Descriptive  Petrology. 


FIG.  85. 


variable,  but  the  percentage  of  silica  is  pretty  constant. 
Before  the  blowpipe  tachylyte  fuses  quite  easily  *  with  intu- 
mescence to  a  dark  slaggy  glass.  It  is  decomposed  with 
gelatinisation  in  hydrochloric  acid.  Its  hardness  is  about 
6'5,  and  its  specific  gravity  about  2*5. 

Under  the  microscope  tachylytes  vary  greatly  in  appear- 
ance, some  being  comparatively  translucent,  at  all  events  in 
places,  while  others  are  almost  wholly  opaque,  even  in  ex- 
cessively thin  sections. 

This  opacity  seems  in  many  cases  to  be  due  to  the  pre- 
sence of  fine  opaque,  black,  dusty  matter  which  pervades  a 

great  portion  of  the 
glass,  but  is  more 
densely  segregated 
around  certain  spots, 
these  spots  being 
frequently  small  cry- 
stals of  magnetite. 
Fig.  85  represents 
part  of  a  section  of 
tachylyte  from  Slie- 
venalargy,Co.  Down, 
Ireland  (magnified 
300  diameters),  in 
which  these  dust  ac- 
cumulations are  well 
shown.  Where  these 
dust  segregations  are  less  dense,  the  sections  often  appear  of  a 
brownish  colour,  and  this  is  possibly  due  to  peroxidation, 
the  magnetite  probably  being  converted  into  martite,  or 
some  closely  allied  mineral.  In  the  clearer  portions  of  the 
same  section  from  which  fig.  85  was  drawn,  numerous  opaque 
patches  of  irregular  form  are  visible  (fig.  86,  magnified  55 
diameters) ;  their  boundaries  are  sharply  defined,  while  they 


Whence  the  name  from  TO.XVS,  quickly,  and  Aurbs,  fusible. 


TacJiylyte. 


201 


are  bordered  by  the  clear  absorption  spaces  from  which  the 
dust,    so    finely  dis- 
seminated     through 
the  other  portions  of 
the  glass,  appears  to 
have   been    abstrac- 
ted.    At  A  A  in  the 
same     drawing     la- 
cunse  of  greenish  co- 
loured     ferruginous 
glass     are      shown, 
which      mineralogi- 
cally    are     possibly 
allied  to  glauconite. 
At  B  B  B  portions  of 
fine  rod -like  bodies 
are   delineated,  and 
these  also  contain,  or  are  fringed  with,  green  matter.     In 
some  places,  as  in  the  central  part  of  the  figure,  a.  tendency 
to  perlitic  structure  may  be  detected.1      A  tachylyte  from 
Bobenhausen  is  described  by  Vogelsang2  as  a  brownish- 
red  glass,  containing  dark  crystallites  which  are  developed 
in  fern-like  forms,  but  which  in  most  parts  of  his  drawing 
appear  to  be  slightly-fringed  spherical  or  cruciform  bodies, 
or  irregular  dark  patches.     He  did  not  regard  this  substance 
as  magnetite,   arguing  that,   when  magnified  800  to  1,000 
diameters,  the  granules  constituting  the  thinner  portions  and 
edges  of  these  crystallites  do  not  appear  opaque  like  mag- 
netite, but  somewhat  translucent  and  of  a  brownish  colour. 
The  glass  around  them  appears  much  clearer  and  of  a  yellow 
colour.     Vogelsang,  Zirkel,  and  Mohl  all  concur  in  regard- 
ing these  bodies  as  similar  to  those  which  often  occur  in 

1  Fuller  details  respecting  this  rock  will  be  found  in  a  paper  by  the 
author  '  On  Microscopic  Structures  in  Tachylyte  from  Slievenalargy, 
Co.  Down,'  Journ.  Royal  Geol.  Soc.  Ireland,  vol.  iv.  part  4,  new  series, 
p.  227. 

•  Die  Krystalliten.     Bonn,  1875,  P-  I1[I' 


2O2  Descriptive  Petrology. 

blast-furnace  slags.  Both  Vogelsang  and  Zirkel  consider 
these  crystallites  to  consist  of  ferruginous  glass,  while  Mohl 
regards  them  as  magnetite. 

The  microscopic  characters  of  several  tachylytes  are  re- 
corded by  Zirkel  in  his  '  Untersuchungen  iiber  die  Basalt- 
gesteine,'  Bonn,  1870,  p.  182.  In  one  from  Meinzereichen 
in  Hesse,  he  cites  the  occurrence  of  fern-like  developments 
around  a  transverse  section  of  an  apatite  crystal.  In  a  sec- 
tion of  tachylyte  from  Some  in  the  Isle  of  Mull,  the  whole 
of  the  section,  although  an  excessively  thin  one,  appears 
opaque  or  very  feebly  translucent  on  the  edges,  while  em- 
bedded in  it  are  minute  transparent  crystals  which  frequently 
show  hexagonal  sections,  and  which  are  probably  apatite. 
Mohl,  in  his  'Basalte  und  Phonolithe  Sachsens,'  Dresden, 
1873,  figures  and  describes  the  occurrence  of  patches  of  tachy- 
lytic  glass  in  sections  of  nepheline-basalt,  which  contain  fern- 
like  trichites,  and  occasionally  small  crystals  of  nepheline. 
Rosenbusch  describes  a  tachylyte  from  Czertochin  in 
Bohemia,  as  a  greenish-grey  glass  full  of  strings  of  minute 
steam  pores  which  occasionally  anastomose.  He  states  that 
this  rock  is  very  quickly  and  completely  dissolved  in  hydro- 
chloric acid,  without  the  application  of  heat.1 


CHAPTER   XII. 

ERUPTIVE   ROCKS. 

CLASS  II. — CRYSTALLINE  ROCKS. 

GRANITE  GROUP. 

Granite. — The  granites  (from  the  Latin  granum,  a  grain) 
are  essentially,  as  their  name  implies,  crystalline-granular 
rocks  which  may  be  regarded  as  consisting  typically  of 

1  Mik.  Physiog.  Min.  p.  139,  fig.  15.     Stuttgart,  1873. 


Granite.  203 

orthoclase,  quartz,  and  mica.  There  is,  however,  considerable 
variation  in  the  mineral  constitution  of  granites.  In  some 
the  felspathic  component  is  not  merely  orthoclase ;  plagio- 
clastic  felspars  such  as  albite  and  oligoclase  being  frequently 
present,  while  the  mica,  which  is  usually  muscovite  or  biotite, 
may  at  times  be  represented  by  lepidolite,  lepidomelane,  or 
other  micas.  Other  minerals,  which  are  not  regarded  as 
essential  constituents  of  granite,  are  often  present.  When 
widely  disseminated,  these  accessory  constituents  play  only 
a  very  subordinate  part ;  but,  in  certain  limited  areas,  they  are 
often  developed  in  sufficient  quantity  to  impart  a  distinctive 
character  to  the  rock.  Thus,  for  example,  schorl  frequently 
occurs  in  considerable  quantity  in  granitic  masses  at  or  near 
their  contact  with  other  rocks,  sometimes  to  such  an  extent 
that  the  term  schorlaceous  granite  is  applied  to  the  rock. 
Apatite  and  magnetite  are  also  minerals  of  common  occur- 
rence in  granites.  Kpidote  and  garnets  are  less  common, 
but  are  often  met  with.  Pyrites  is  common  in  many  granites, 
and  it  seems  doubtful  whether,  in  some  cases,  it  should  be 
regarded  as  a  mineral  of  secondary  origin.  Talc,  beryl, 
iolite,  andalusite,  topaz,  cassiterite,  and  hematite  are  oc- 
casionally met  with  in  granitic  rocks.  Hornblende  is  of 
common  occurrence,  and,  when  tolerably  plentiful,  the  rock 
is  then  termed  hornblendic  or  syenitic  granite.  When  quartz 
is  absent,  or  only  poorly  represented,  and  the  mica  is  replaced 
by  hornblende,  the  rock  is  called  syenite.1  Chlorite,  epidote, 
pinite,  and  several  other  products  of  the  alteration  of  other 
minerals,  are  not  of  unfrequent  occurrence  in  these  rocks. 

Kaolin  very  commonly  occurs  in  granites,  and  results 
from  the  decomposition  of  the  felspars.  Graphite  is  also 
met  with  at  times. 

Granite  rocks  vary  very  considerably  in  texture  and  in 
structural  characters. 

1  It  should,  however,  be  remarked  that  the  term  syenite,  as  first 
employed  by  Pliny,  and  as  used  in  most  geological  works,  until  within 
the  last  few  years,  implied  hornblendic  granite,  such  as  that  which 
comes  from  the  quarries  of  Syene  in  Egypt. 


204  Descriptive  Petrology. 

The  granites,  as  a  rule,  are  either  coarsely  or  finely 
crystalline-granular  in  texture,  and,  when  very  fine-grained, 
and  the  mica  is  only  poorly  represented,  or  totally  absent, 
pass  into  felstones  of  variable  texture.1  The  granitic  rocks 
are  frequently  porphyritic.  Crystals  of  orthoclase  several 
inches  in  length  being  of  common  occurrence  in  some,  while, 
in  others,  the  mica  (usually  muscovite)  forms,  by  its  con- 
spicuous development,  the  dominant  mineral.  When  mica 
is  scarce,  and  the  rock  assumes  a  felsitic  character,  it  is 
common  to  find  eicher  orthoclase  or  quartz  porphyritically 
developed.  In  the  former  case  the  rock  would  be  styled  a 
felspar  porphyry,  in  the  latter  a  quartz-porphyry,  or  elvan.2 

In  true  granites  no  microrcrystalline  or  amorphous  paste 
is  visible  between  the  crystals  and  crystalline  grains  of 
which  the  rock  is  composed.  Of  these  component  minerals 
orthoclase  is  generally  the  most  plentiful;  next  follows 
quartz,  and  then  mica,  in  the  usual,  but  not  the  invariable, 
order  of  quantitative  importance.  In  their  order  of  solidi- 
fication, or  crystallisation,  quartz  appears  to  come  last,  and,- 
although  it  sometimes  occurs  in  hexagonal  pyramids  or  in 
combinations  of  the  pyramid  and  prism,  still  its  development, 
as  a  rule,  seems  to  have  been  imperfect,  and  to  have  resulted 
mainly  in  irregularly  shaped,  angular,  crystalline  grains.  The 
orthoclase  in  granites  varies  in  colour.  In  some  it  is  red, 
often  of  a  flesh-red  or  pink  tint,  in  others  white,  grey,  or 
yellowish.  It  very  commonly  occurs  in  Carlsbad  twins. 
The  crystals  are  often  several  inches  in  length,  as  in  some 
of  the  Dartmoor  granites. 

When  granites  are  weathered,  the  felspar  crystals  are 
converted  into  kaolin  and  the  rock  in  course  of  time 
crumbles  away.  The  kaolin  or  china-clay  which  remains 

1  Felsitic  matter,  which  constitutes  the  chief  bulk  of  felstones,  is  a 
very  finely  crystalline-granular,  or  micro-crystalline,  or  crypto-crystal- 
line,  admixture  of  orthoclase  and  quartz. 

2  Elvan  is  a  Cornish  name,  and  is  commonly  applied  by  the  Cornish 
miners  to  most  of  the  dykes  which  occur  in  that  county,  irrespective  of 
their  mineral  constitution.     The  term  has,  however,  of  late  years  been 
restricted  to  quartz-porphyries. 


Granite.  205 

after  the  disintegration  of  granite  frequently  contains  a  large 
proportion  of  quartz  grains,  and  this  decomposed  rock  is 
known  as  china-stone. 

The  crystals  of  orthoclase  are  not  always  well  developed 
in  granites ;  they  sometimes  have  very  irregular  contours, 
and  occasionally  their  angles  are  rounded.  Under  the 
microscope  they  frequently  present  a  more  or  less  turbid 
appearance,  and  this  greatly  increases  in  proportion  to  the 
stage  of  decomposition  at  which  the  rock  has  arrived, 
until  they  ultimately  become  completely  kaolinised  and 
opaque.  They  occasionally,  but  rarely,  contain  fluid 
lacunae.  Plagioclastic  felspars,  either  albite  or  oligoclase, 
are  of  frequent  occurrence  in  granites.  They  usually  occur 
in  smaller  crystals  than  the  orthoclase.  Under  the  micro- 
scope they  exhibit,  when  fresh,  the  characteristic  twinning 
of  plagioclase,  but,  as  decomposition  advances,  a  granulated 
structure  also  supervenes,  which  obliterates  this  distinctive 
structure,  and  renders  it  impossible  to  determine  whether 
they  were  originally  monoclinic  or  triclinic  felspars. 

The  quartz,  as  already  stated,  sometimes  occurs  in  well- 
developed  crystals,  and  sometimes  in  angular,  crystalline 
grains.  The  former  often  exhibit  a  polysynthetic  structure 
when  examined  in  polarised  light.  Under  the  microscope,  in 
thin  sections,  they  appear  quite  glassy  and  clear,  and  are 
seen  to  contain  numerous  fluid  lacunae,  which  are  often  so 
plentiful,  as  to  impart  an  almost  turbid  appearance  to  the 
crystal  or  granule.  The  contained  fluid  is  generally  water 
or  aqueous  solutions  of  chlorides  and  sulphates  of  sodium, 
potassium,  and  calcium.  Apatite  crystals  are  also  frequently 
visible  in  the  quartz  of  granite.  The  micas  in  thin  sections 
of  granite  appear  either  in  well  denned  crystals,  which, 
when  the  section  is  taken  parallel  to  their  basal  planes, 
appear  as  six-sided  tables,  or  in  scales  of  irregular  form. 
The  potash  micas  appear  clear  and  nearly  colourless,  while 
the  magnesian  micas  are  dark  reddish-brown  or  black,  and 
the  latter  show  strong  dichroism,  when  the  planes  of  section 
do  not  coincide  with  the  basal  planes  of  the  crystals. 


206  Descriptive  Petrology. 

When  schorl  occurs  in  granites  it  may  usually  be  recog- 
nised by  the  strong  bluish  tint  which  it  here  and  there  shows, 
when  examined  under  the  microscope  by  ordinary  transmit- 
ted light,  and  also  by  the  approximately  triangular  transverse 
sections  which  the  crystals  frequently  exhibit. 

When  in  thin  sections  of  granite,  magnetite  and  pyrites  are 
present,  they  both  appear  opaque,  and,  to  distinguish  between 
them,  it  is  necessary  to  examine  them  by  reflected  light, 
when  their  differences  of  colour  and  lustre  become  apparent. 

With  regard  to  the  origin  of  granite,  there  has  been  con- 
siderable discussion,  in  which  most  antagonistic  opinions 
have  been  brought  forward ;  theories  of  its  igneous,  aqueous, 
and  metamorphic  origin  having  all  been  strongly  advocated. 
Its  eruptive  character  is  inferred  from  the  granite  veins, 
which  in  certain  localities  traverse  older  rocks  in  a  most 
irregular  manner,  while  the  dykes  of  quartz-porphyry  which 
often  emanate  from,  and  can  be  traced  to  underlying  granitic 
masses,  and  are,  indeed,  mere  differentiations  of  granite, 
afford  additional  proofs  of  its  eruptive  character. 

According  to  Hermann  Credner,  however,  the  mineral 
matter  of  the  granitic  veins  in  Saxony  is  not  derived  from 
deep  sources,  but  from  the  partial  decomposition  of  the 
adjacent  rocks  by  the  infiltration  of  water,  and  he  observes 
that  the  mineral  characters  of  the  veins  are  influenced  by 
those  of  the  rocks  which  they  traverse. * 

With  regard  to  the  larger  bosses  and  the  huge  granitic 
masses,  from  which  such  dykes  and  veins  are  given  off,  we 
can  scarcely  deny  to  the  parent  masses  the  origin  which 
must  be  attributed  to  their  offshoots,  but,  in  the,  absence  of 
such  veins  and  dykes,  it  is  easy  to  understand  how,  with 
considerable  show  of  reason,  a  metamorphic  origin  may  be 
assigned  to  those  masses  which,  though  once  deep-seated, 
are  now  exposed  by  the  denudation  of  enormous  thicknesses 
of  once  overlying  rock,  and  the  question  rather  naturally 

1  '  Die  Granitischen  Gange  des  sachsischen  Granulitgebirges,'  Her- 
mann Credner,  Zeitsch.d.  deutsch.  geol.  Ces.,  Jahrg.  1875,  p.  218. 


Granite.  207 

arises  whether  they  have  not  resulted  from  the  metamor- 
phism  of  sedimentary  deposits,  once  so  far  beneath  the 
earth's  surface  that  they  lay  within  a  zone  of  comparatively 
high  temperature.  Admitting  this,  it  seems  that  we  are  ad- 
mitting no  more  than  the  conditions,  or  phases  of  the  condi- 
tions, under  which  all  eruptive  rocks  have  been  formed,  and 
which  are,  therefore,  just  as  fully  entitled  to  the  appellation 
of  metamorphic  rocks.  That  the  passage  sometimes  observed 
from  granite  into  gneiss  is  a  proof  that  granite  is  the  extreme 
phase  of  the  metamorphism  of  sedimentary  rocks  does  not 
always  appear  to  be  conclusive,  since  instances  are  known  in 
which  foliation  is  not  indicative  of  bedding,  and  a  few  cases 
are  recorded  in  which  gneiss  actually  occurs  in  veins.  In 
the  present  conflicting  state  of  opinion  upon  this  subject  it 
behoves  examination  candidates  to  accept  and  cite  the 
different  opinions  commonly  held  and  set  forth  in  the 
various  manuals  of  geology.  The  student  may  afterwards 
judge  of  their  respective  merits  from  his  own  observations. 

One  of  the  arguments  against  the  igneous  origin  of 
granite  is  that  in  granite  the  quartz  has  a  specific  gravity  of 
2-6,  identical  with  that  of  silica  derived  from  aqueous  solu- 
tion, while  the  specific  gravity  of  fused  silica  is  only  2*2. 

This  observation,  in  conjunction  with  many  others, 
appears  to  have  influenced  to  some  extent  the  deductions  of 
Professor  Haughton,  in  his  annual  address  to  the  Geological 
Society  of  Dublin  in  1862.  After  giving  a  table  of  the 
relative  specific  gravities  of  natural  and  artificially  fused 
rocks,  he  concludes  in  the  following  words  : — 

'  It  appears  to  me  that  the  column  of  differences '  (in 
the  specific  gravities  of  natural  and  artificially-fused  rocks) 
'greatly  strengthens  the  argument  of  those  chemists  and 
geologists  who  believe  that  water  played  a  much  more  im- 
portant part  in  the  formation  of  granites  and  traps  than  it  has 
done  in  the  production  of  trachytes,  basalts,  and  lavas,  and  that 
they  owe  their  relatively  high  specific  gravity  to  its  agency.' 

'The  only  manner  in  which  it  seems  possible  to  reconcile 


2o8  Descriptive  Petrology. 

the  opposite  theories  of  the  origin  of  granite,  derived  from 
physical  and  chemical  arguments,  is  to  admit  for  granite 
what  may  be  called  hydro-metamorphic  origin,  which  is  the 
converse  of  what  is  commonly  called  metamorphic  action, 
but  which  might  more  properly  be  designated  pyro- metamor- 
phic action.  The  metamorphism  of  rocks  might  thus  be 
assumed  to  be  twofold.  Hydro-metamorphism,  by  which 
rocks,  originally  fused,  and  when  in  liquid  fusion,  poured 
into  veins  and  dykes  in  pre-existing  rocks,  are  subsequently 
altered  in  specific  gravity  and  arrangement  of  minerals,  by 
the  action  of  water  acting  at  temperatures  which,  though 
still  high,  would  be  quite  inadequate  to  fuse  the  rock  ;  and 
pyro-metamorphism,  by  which  rocks  originally  stratified  by 
mechanical  deposition  from  water,  come  to  be  subsequently 
acted  on  by  heat,  and  so  transformed  into  what  are  com- 
monly called  the  metamorphic  rocks.' 

'  Granite,  it  appears  to  me,  although  generally  a  hydro- 
metamorphic  rock,  may  occasionally  be  the  result  of  pyro- 
metamorphic  action  ;  and  such  appears  to  have  been  its 
origin  in  Donegal,  in  Norway,  and,  perhaps,  in  the  chain  of 
the  Swiss  Alps.' l 

This  may  be  a  very  just  opinion,  especially  if  Professor 
Haughton  does  not  imply,  in  his  pyro-metamorphic  action, 
the  total  exclusion  of  water  from  any  participation  in  the 
changes  effected.  The  two  conditions  of  metamorphism 
which  he  indicates,  most  likely  represent;  in  the  hydro- 
metamorphism,  the  presence  of  a  large  proportion  of  water 
and  a  moderately  high  temperature  ;  in  the  pyro-  metamor- 
phism, a  comparatively  small  portion  of  water  and  a  much 
higher  temperature.  Such  at  least  is  a  probable  construction 
to  put  upon  these  conclusions ;  but,  so  far  as  metamorphism 
in  its  vulgar  acceptation  is  concerned,  there  seems  no  reason, 
apart  from  the  distinctions  just  given,  for  regarding  granite 

1  '  On  the  Origin  of  Granite, '  an  address  delivered  before  the  Geo- 
logical Society  of  Dublin,  by  the  Rev.  Samuel  Haughton,  F.R.S. 
Dublin,  1862. 


Granite.  209 

as  a  metamorphic  rock  any  more  than  basalt  or  trachyte, 
which  have,  in  a  certain  sense,  resulted  from  the  extreme 
alteration  of  other  rocks.  The  crystalline  schists,  gneiss, 
&c.,  are  but  phases  of  the  conversion  of  sediments  into  true 
eruptive  rocks  ;  and,  if  the  degree  of  alteration  be  put  out 
of  the  question,  the  crystalline  schists,  the  plutonic  rocks, 
and  the  volcanic  rocks,  all  seem  equally  eligible  for  the  term 
metamorphic.  In  questions  of  metamorphism,  it  appears 
that  the  nature  of  the  change  is  the  first  thing  to  consider  ; 
its  cause,  the  next;  its  degree,  the  last.1 

The  different  varieties  of  granite  and  of  granitoid  rocks 
may  be  summed  up  under  the  following  heads  : — 

Porphyritic  granite,  in  which  the  felspar  crystals  are 
large  and  well  developed,  being  frequently  several  inches  in 
diameter,  as  in  those  of  Cornwall,  Dartmoor,  Shap,  &c. 

Various  grades  of  texture  occur  between  these  granites 
and  those  which  are  termed  fine-grained.  When  of  the 
latter  character,  they  pass  into  micaceous  felstones. 

Felstone  (eurite,2  halleflinta,  petrosilex),  consists  of  felsitic 
matter,  (viz.,  an  intimate  granular-crystalline,  micro-crystal- 
line, or  crypto-crystalline,  admixture  of  orthoclase  and 
quartz,  in  which  crystalline  granules  of  plagioclastic  felspars 
not  unfrequently  occur.)  In  this  felsitic  base,  which,  typi- 
cally," constitutes  the  matrix  of  all  felstones,  felspar  crystals, 
commonly  orthoclase,  are  often  developed  ;  and,  like  those 
in  the  porphyritic  granites,  are  frequently  twinned  on  the 
Carlsbad  type.  Such  rocks  are  termed  felspar  porphyries . 

1  The  less  the  unqualified  term  metamorphism  is  used,  the  better ; 
since  it  merely  implies  change,  without  specifying  the  nature  or  extent 
of  the  change  or  the  conditions  under  which  the  change  took  place. 

2  The  terms  Felstone  and  Eurite  are  frequently  used  synonymously ; 
but  eurite  is  stated  by  some  authors  to  be  more  easily  fusible  than 
orthoclase,  while  the  eurite  snrsilicee  of  Cordier   is   more   difficultly 
fusible.     The  name  Eurite  is  due  to  d'Aubuisson.     Kinahan's  definition 
of  eurite,  as  a  basic  felstone  (Handy-Book  of  Rock  Names,  p.  48),  might 
lead  the  unwary  to  regard  it  as  a  rock  containing  less  than  60  per  cent. 
of  silica,  but  he  is  probably,  to  some  extent,  right  in  keeping  the  dis- 
tinction between  eurite  and  felstone,  although  at  times  rocks  of  an 
intermediate  character  are  met  with. 


2 1  o  Descriptive  Petrology. 

If,  in  such  a  felsitic  matrix  quartz  occurs  porphyritically 
either  in  crystals,  but  more  usually  in  roundish  blebs,  the 
rock  is  termed  quartz-porphyry  ;  but  it  is  common  to  find 
porphyritic  orthoclase  crystals  also  developed  in  quartz- 
porphyries.  It  seems,  however,  probable  that  the  ground - 
mass  of  true  quartz-porphyries  should,  in  many  cases,  rather 
be  regarded  as  a  very  fine-grained  or  micro -crystalline  granite. 
Rocks  of  this  class  are  called  elvans  by  the  Cornish  miners, 
and,  indeed,  in  that  district,  the  term  elvan  is  very  loosely 
applied.  As,  however,  the  dyke-forming  rocks  of  Cornwall 
are  mostly  offshoots  from  the  granitic  masses  of  that  district, 
the  term  elvan  has  for  the  most  part  been  applied  to  more 
or  less  fine-grained  or  porphyritic  granitoid  rocks,  and  it  is 
now,  as  a  rule,  regarded  as  a  synonym  for  quartz-porphyry, 
or,  as  some  authors  term  it,  quartz- felsite. 

Granitite  is  a  term  given  to  those  varieties  of  granite 
which  contain  a  certain  amount  of  plagioclase  (oligoclase). 
The  orthoclase,  in  the  rock  to  which  this  name  has 
been  applied,  is  flesh -red,  and  this  mineral  and  quartz  are 
the  two  principal  constituents.  The  mica  is  a  blackish- 
green  magnesian  mica,  but  it  is  usually  present  only  in  small 
quantity. 

Since  plagioclastic  felspars  exist,  though  in  a  subordinate 
capacity,  in  many  granites,  it  seems  that  no  line  of  demarca 
tion  can  be  drawn  between  them  and  the  granitites. 

Cor dier tie-granite  is  a  variety  occurring  in  certain  localities 
in  Norway,  Greenland,  and  Bavaria.  It  is  characterised  by 
containing  cordierite  or  iolite ;  this  mineral  partially,  and 
sometimes  wholly,  replacing  the  mica.  A  greenish  oligoclase 
is  often  present  in  the  rock. 

Luxullianite  is  composed  of  schorl,  flesh-coloured  ortho- 
clase and  quartz.  The  schorl,  which  is  black,  or  greenish- 
black,  is  distributed  in  irregular  nests  or  patches,  and 
contrasts  strongly  in  colour  with  the  other  constituents 
of  the  rock.  Boulders  of  this  stone  occur  in  the  neigh- 
bourhood of  Luxullian,  in  Cornwall  ;  and  the  late  Duke 


Granulite.     Greisen.      Gneiss.  2 1 1 

of  Wellington's  sarcophagus  was  made  from  one  of  them. 
The  rock  has  not,  however,  been  met  with  in  situ. 

In  a  paper  by  Professor  Bonney,  published  in  No.  7  of 
the  '  Mineralogical  Magazine/  1877,  two  varieties  of  tourma- 
line are  stated  to  occur  in  this  rock,  and  some  evidence  is 
adduced  to  show  that  this  mineral  is  a  product  of  alteration. 

Aplite  or  Haplite  (from  aTrXooc,  simple),  also  termed 
semi"-granite  (Halb-granif]  or  granitell,  is  a  rock  of  limited 
occurrence,  consisting  of  a  crystalline-granular  admixture 
of  felspar  and  quartz.  The  so-called  graphic-granite  or 
pegmatite  is  a  structural  variety  of  this  rock,  in  which  the 
quartz  is  developed  in  such  a  manner  that  it  roughly 
resembles  Hebrew  characters,  a  polished  surface  of  the  rock 
appearing  closely  inscribed,  whence  the  name  'graphic.' 

Granulitt  (Weiss-stein  or  leptinite)  is  also  composed  of 
felspar  and  quartz,  the  felspar  being  orthoclase.  In  structure 
it  is  more  or  less  finely  crystalline-granular,  and  frequently 
has  a  foliated  or  schistose  character.  It  generally  con- 
tains numerous  garnets,  which,  in  thin  section  under  the 
microscope,  appear  as  little  irregular,  roundish,  singly- 
refracting  grains,  like  drops  of  gum.  Mr.  John  Arthur 
Phillips  has  observed  double  refraction  in  the  garnets  of 
some  granulites. 

This  rock,  in  its  schistose  structure  and  mode  of 
occurrence,  seems  to  bear  much  the  same  relation  to  felstone 
that  gneiss  bears  to  granite,  and  it  may  therefore  be  classed 
with  the  metamorphic  rocks.  It  often  contains  schorl  and 
hornblende  microliths,  and  occasionally  sphene.  The 
variety  called  trap-granulite  contains  plagioclastic  felspars, 
and  is  somewhat  poorer  in  silica. 

Greisen  (Zwitter,  Stockwerks-porphyr}  is  a  granular- 
crystalline  rock,  consisting  of  quartz  and  mica,  the  latter 
usually  lithia-mica.  Quartz  is,  however,  the  predominating 
constituent.  When  orthoclase  occurs  in  it  the  rock  passes 
into  granite.  Tinstone  (cassiterite)  is  very  commonly  met 
with  in  greisen,  either  in  small  strings  and  veins,  or  in  little 

p  2 


2 1 2  Descriptive  Petrology. 

crystals  or  granules.  It  is  a  rock  of  common  occurrence 
in  Saxony  and  Cornwall.1 

Gneiss. — This  term,  in  its  proper  sense,  signifies  foliated 
granite  ;  but  foliated  rocks,  consisting  to  a  very  great  extent 
of  hornblende  and  quartz,  have  also  been  styled  gneiss, 
although  they  should  rather  be  termed  schistose  amphibolites. 
Indeed,  the  name  seems  to  have  been  somewhat  loosely 
applied  to  foliated  crystalline  rocks  of  variable  mineral 
constitution. 

True  gneiss  differs  in  no  way  from  granite,  except 
structurally.  A  foliated  structure  is  its  essential  peculiarity. 
It  is  sometimes  interbedded  with  other  rocks,  and  frequently 
exhibits  stratification,  which  is  often  but  not  invariably 
coincident  with  the  foliation.  Darwin  has  shown  that,  in 
the  gneiss  of  the  Andes,  the  planes  of  foliation  coincide 
with  planes  of  cleavage.  Sir  R.  I.  Murchison  pointed  out 
that  the  foliation  in  some  of  the  Scotch  Silurian  rocks  cor- 
responds with  the  planes  of  bedding  ;  and  similar  observa- 
tions have  been  made  in  Anglesey,  by  Professor  Henslow ; 
while,  according  to  Professor  Ramsay,  and  a  host  of  other 
observers,  the  coincidence  of  foliation  with  bedding  is  of 
extremely  common  occurrence.  To  crystalline  rocks,  which 
exhibit  this  structure,  the  adjective  gneissic  is  applied,  a  good 
practice,  when  the  rock  deviates  in  mineral  composition  from 
a  true  granite. 

Gneiss  has  been  split  up  into  numerous  varieties,  which, 
m  the  main,  are  identical  in  mineral  constitution  with  the 
corresponding  varieties  of  granite.  Thus  we  have,  in  addi- 
tion to  ordinary  gneiss,  oligoclase  gneiss,  a  foliated  rock 
corresponding  with  oligoclase  granite,  dichroite  gneiss, 
adularia  gneiss,  garnet  gneiss,  syenitic  gneiss,  &c.,  &c. 

Protogine  is  a  gneiss  in  which,  in  addition  to  the  ordinary 
constituents  of  granite,  a  greenish,  pearly,  or  silvery  talcose 

1  A  paper,  'On  some  of  the  Stock  works  of  Cornwall,'  by  Dr.  C. 
Le  Neve  Foster,  was  read  before  the  Geological  Society,  London, 
January  9,  1878. 


Deviations  from  the  Granitic  Type.          2 1 3 

mineral  is  present.  The  rock,  when  not  foliated,  is  termed 
protogine  granite. 

Cornubianite  (proteolite)  is  a  compact  granular- scaly  con- 
dition of  gneiss,  which  is  met  with,  at  times,  at  the  contact 
of  granites  with  slates. 

The  accessory  mineral  constituents  which  occur  in 
gneiss  are  very  numerous,  and  are  similar  to  those  which 
occur  as  accessories  in  granites. 

Those  rocks  which  in  mineral  constitution  and  in  struc- 
ture more  or  less  resemble  granites,  are  spoken  of  as  granitoid 
rocks.  They  range  from  the  coarsely  crystalline  to  the 
micro- crystalline  or  crystalline-granular  varieties. 

Many  of  them  have  a  felsitic  matrix,  their  distinctive 
characters  being  due  to  the  larger,  or  porphyritic,  develop- 
ment of  one  or  more  of  their  mineral  constituents. 

The  felsitic  matrix  of  these  rocks  consists  of  an  intimate 
micro-crystalline,  or  granular  admixture  of  felspar  (mostly 
orthoclase)  and  quartz. 

The  following  table  might  be  greatly  extended  so  as  to 
embrace,  all  the  chief  rocks,  both  basic  and  highly  silicated. 
Thus,  for  instance,  if  plagioclastic  felspar  were  substituted 
for  orthoclase,  then  syenite  would  become  diorite.  The 
student,  by  constructing  such  tables,  may  thus,  as  his 
knowledge  increases,  see  how  far  the  classification  of  rocks 
is  useful,  and  how  they  gradually  pass  from  one  type  to 
another. 

Gneiss,  granulite,  and  several  other  rocks,  have  been 
described  in  this  place  because  they  are  closely  related  to 
granite  in  mineral  constitution  ;  but  they  should,  perhaps, 
in  most  cases,  rather  be  classed  with  the  metamorphosed 
sedimentary  rocks. 

TABULAR  VIEW  OF  SOME  OF  THE   PRINCIPAL  DEVIATIONS 
FROM  THE  GRANITIC  TYPE. 

In  this  table  the  letter  F  indicates  felspar  of  one  or  more 


2 1 4  Descriptive  Petrology. 

species,  but  mostly  orthoclase.     M  represents  mica  of  one  or 
more  species.     Q  =  quartz.     H  =  hornblende. 

Syenite                .  F  H    cryst-granular. 

Quartz- syenite    .  F  Q  H    cryst-gran. 

Syenitic  granite  .  F  M    Q  H )  cryst-gran. 

Syenitic  gneiss   .  F  M    Q  H  j  foliated 

GRANITE         .  F  M    Q  \ cryst-gran. 

GNEISS   .         .  F  M    Q  [foliated. 

Haplite               .  F  Q  |  cryst-gran. 

Granulite  F  Q  j  schistose. 

Quartz-porphyry  F  O  \  Felsitic  matrix — O  porphyritic. 

Felspar-porphyry  F  Q  „            „         F          „ 

FeMone             .  F  Q,  „            „ 

Greisen      .  M    Q  cryst-gran. 

Ouartzite    .         .  Q  gran,  compact. 

FELSTONE  GROUP. 

Felstone  (eurite,  petrosilex,  halleflinte,  felsite). — Fel- 
stone  is  a  more  or  less  compact  rock,  those  varieties  termed 
halleflinte  and  hornstone  having  a  peculiarly  flinty  aspect, 
while,  in  other  cases,  the  rock  is  either  finely  crystalline- 
granular  or  granular,  sometimes  porphyritic,  often  micro- 
porphyritic.  In  colour  felstone  varies  very  greatly — brick- 
red,  brown,  grey,  yellowish,  and  greyish-white  tints  being 
the  most  common.  Many  varieties  have  a  more  or  less 
conchoidal  fracture,  and  all  of  them,  before  the  blowpipe, 
are  fusible  on  the  edges  of  splinters  to  a  white  or  speckled 
enamel.  The  eurites  proper  are  more  easily  fusible  than 
the  felstones  or  eurites  sursilicees  of  Cordier.  In  the  com- 
pact and  in  the  non-porphyritic  examples  no  definite  mine- 
rals can  be  detected  with  the  naked  eye  or  with  a  lens,  and 
the  same  may  be  said  of  the  matrix  in  which  porphyritic 
crystals  occur.  Sometimes,  but  not  commonly,  they  present 
an  imperfect  schistose  structure,  as  in  the  varieties  termed 
felsite  schist.  They  differ  considerably  in  chemical  compo- 
sition, the  amount  of  silica  which  they  contain  varying  from 


Deviations  front  Granite  as  a  Type. 

PLATE  111. 


215 


2 1 6  Descriptive  Petrology. 

about  70  to  80  per  cent.,  and  they  frequently  have  about  5 
per  cent,  of  the  alkalies.  Under  the  microscope  they  are 
also  seen  to  vaiy  greatly  in  character.  Sometimes  they  show 
a  micro-crystalline  structure,  in  which,  by  polarised  light,  the 
section  breaks  up  into  a  many-coloured  mosaic,  and  the 
individual  granules  may  be  distinguished  and  identified, 
some  of  them  as  felspars,  others  as  quartz,  in  others  there  is  a 
somewhat  similar  but  less  defined  structure,  crypto-crystalline, 
in  which  individual  minerals  cannot  be  recognised.  In  some 
rare  cases  a  considerable  amount  of  true  vitreous  matter  may 
be  detected,  lying  between  the  micro-crystalline  or  granular 
component  particles,  or  constituting  the  entire  paste.  More 
frequently  the  rock  is  wholly  micro-crystalline  or  micro- 
felsitic.  In  the  latter  case  between  crossed  Nicols  the  sub- 
stance behaves  as  an  amorphous  mass.  In  this  case  the 
structure  may  be  granular,  fibrous,  or  microlitic,  the  granular 
and  fibrous  structure  seldom  presenting  any  definite  character 
or  individualisation  of  the  constituent  granules  and  fibres. 
Sections  of  such  a  rock  dc  not  however  always  present  total 
obscurity  between  crossed  Nicols,  but  transmit  a  feeble  light, 
in  an  irregular  and  fickle  manner,  as  regards  its  distribution. 
Sections  of  felstone  occasionally  present  a  radial-fibrous 
structure  under  the  microscope  ;  these  constitute  the  varieties 
known  as  spherulitic  felsite.  Fluxion-structure  is  sometimes 
to  be  observed  in  felstones.  It  is  probable,  however,  that 
many  of  the  felsitic  rocks  which  show  this  are  more  or 
less  closely  allied  to  the  rhyolites.  •  It  is  quite  possible  that 
in  many  cases  the  micro-crystalline  or  micro-granular  struc- 
ture of  felstones  simply  represents  the  devitrification  of  an 
originally  glassy  magma,  but,  as  remarked  by  A.  von 
Lasaulx,  the  felsite  pitchstones  frequency  fail  to  present  the 
microscopic  structure  so  characteristic  of  felstones.  It  is 
nevertheless  far  from  uncommon  to  find  small  patches  in 
sections  of  pitchstones  and  other  vitreous  rocks,  in  which 
devitrification  has  resulted  in  the  production  of  structure, 
strikingly  analogous,  if  not  identical,  witli  that  of  felstones. 


Syenite,  217 

Hornblende,  micas,  sometimes  potash,  sometimes  magnesian, 
magnetite,  titaniferous  iron,  &c.,  are  met  with  in  felstones. 
Felspars  are,  also,  often  porphyritically  developed,  and  the 
rock  then  becomes  porphyritic  fel stone  or  felspar  porphyry. 
Quartz  also  occurs  porphyritically  at  times,  either  in  crystals, 
or  in  roundish  grains  ;  the  rock  then  becoming  a  quartz- 
porphyry  or  elvanite. 

Indeed,  passages  may  occur  from  felstone  into  granite, 
syenite,  and  various  rhyolites,  Felstone  is  generally  more 
or  less  porphyritic,  and  occurs  in  dykes,  veins  and  inter- 
bedded  sheets. 

SYENITE  GROUP. 

Syenite,  in  the  acceptation  of  the  term,  as  first  em- 
ployed by  Werner,1  is  a  crystalline-granular  rock,  con- 
taining from  55  to  60  per  cent,  of  silica,  and  consisting 
typically  of  orthoclase  and  hornblende.  In  mineral  consti- 
tution, therefore,  it  approximates  to  some  of  the  trachytes. 
Sometimes  the  felspar  is  microcline,  and  plagioclastic  felspars 
are  nearly  always  present.  Sometimes  augite  or  mica  take 
the  place  of  the  hornblende,  and  occasionally  the  rock  con- 
tains more  or  less  sphene  and  quartz. 

The  syenites  may  therefore  be  divided  into  three  groups, 
viz.,  hornblende  syenite,  augite  syenite,  and  mica  syenite. 
When  quartz  is  present  in  any  notable  quantity  the  reck 
passes  over  to  quartz  syenite,  and  thence,  when  mica  occurs, 
into  syenitic  granite. 

Hornblende  Syenite. — Orthoclase  and  hornblende  are  the 
chief  constituents.  Triclinic  felspar  is  usually  present  in 

1  For  many  years  it  has  been  a  common  practice  to  apply  the  name 
syenite  to  syenitic  or  hornblendic  granite.  At  times  there  has  been 
considerable  difference  of  opinion  about  the  application  of  the  name, 
which  was  first  used  by  Pliny  (Syenites)  for  the  rock  quarried  at  Syene 
in  Egypt.  The  stone  occurring  at  that  locality  is  hornblendic  granite. 
Hornblendic  granite  seems,  therefore,  to  have  a  decided  priority  of 
claim  to  the  name  Syenite,  but  petrologists  have  found  it  convenient 
to  restrict  its  application  to  quartzless  rocks,  such  as  those  here 
described. 


2 1 8  Descriptive  Petrology. 

variable  quantity  ;  it  may  generally  be  referred  to  oligoclase, 
but  the  amount  is  as  a  rule  comparatively  small.  The  colour 
of  the  rock  mostly  depends  upon  the  colour  of  the  orthoclase, 
which  varies  considerably,  red,  brown,  and  white  being  the 
prevailing  colours.  The  orthoclase  crystals  are  frequently 
twinned  on  the  Carlsbad  type.  The  hornblende  is  usually 
greenish-black,  and  the  crystals  are  generally,  but  not 
invariably,  short ;  long  bladed  or  acicular  crystals  sometimes 
occurring.  The  mica  is  a  dark  magnesian  mica,  commonly 
biotite.  Epidote,  magnetite,  sphene,  and  pyrites  frequently 
occur  as  accessories  in  this  rock. 

In  structure  the  syenites  as  a  rule  greatly  resemble 
granites,  and  they  also  occur  in  large  eruptive  masses, 
bosses,  or  veins.  The  gneissic  syenite  sometimes  occurs 
in  considerable  beds,  especially  in  the  Laurentian  series 
of  Canada.  The  foliated  rock  of  Cape  Wrath  in  Suther- 
landshire,  Scotland,  is  rather  amphibolite  schist  than  gneiss, 
and  some  of  the  so-called  gneiss  of  the  Hebrides  may  also 
be  referred  to  hornblendic  schists. 

Augite  Syenite  is  composed  of  felspars,  which,  as  a  rule, 
are  mostly  orthoclastic,  but  the  plagioclastic  ones  occa- 
sionally, though  rarely,  predominate.  Augite  is  frequently 
plentiful,  and  sometimes  a  little  hornblende  occurs,  which, 
as  pointed  out  by  A.  von  Lasaulx,  is  generally  of  a  uralitic 
character,  implying  subsequent  alteration  of  some  of  the 
pyroxenic  constituents.  Biotite,  apatite,  magnetite,  and 
sphene  are  also  of  common  occurrence  as  accessories  in  the 
composition  of  augite  syenite.  According  to  V.  Lasaulx 
sphene  is  less  plentiful  in  those  varieties  in  which  orthoclase 
is  the  predominant  felspar.  Analyses  show  that  the  augite 
syenites  contain  one  or  two  per  cent,  less  of  silica  than  the 
hornblende  syenites. 

Mica  Syenite  is  by  no  means  a  common  rock,  Calabria 
being  almost  the  only  district  in  which  it  is  met  with  to  any 
considerable  extent.  It  occurs  mostly  in  the  form  of  veins 
or  dykes.  The  rock  consists  of  orthoclase,  sometimes 


Minette.  219 

more  or  less  plagioclastic  felspar,  biaxial  magnesian  mica, 
hornblende,  occasionally  some  augite,  which  is  often  altered 
into  pseudomorphs  of  chlorite  or  delessite,  as  in  the  minette 
of  Seifersdorf  in  Saxony,  while  apatite,  calcite,  magnetite,  and 
pyrites  are  also  of  common  occurrence  in  these  rocks  ;  but, 
as  a  rule,  sphene  is  never  met  with  in  mica  syenite.  The 
calcite  and  pyrites  are  products  of  secondary  origin. 

According  to  Rosenbusch,1  Zirkel,  and  other  petrologists, 
mica  syenite  and  minette  are  intimately  related  if  not 
identical.  The  former  author  also  points  out  that  some 
minettes  are  to  be  referred  to  the  augite  syenites. 

Minette, — The  matrix  or  paste  of  minette  appears,  under 
the  microscope,  as  granular  or  granular-crystalline  matter, 
in  which  microliths  frequently  occur ;  the  latter  according  to 
A.  von  Lasaulx,  who  classes  minette  with  the  felstones, 
consist  of  felspar  and  mica,  and  the  preponderance  of  evi- 
dence shows  that  quartz  is  an  exceptional  constituent  of  the 
rock.  Under  these  circumstances  the  matrix  can  hardly 
be  designated  felsitic,  and  upon  this  ground  hinges  the 
question  whether  minette  should  be  classed  with  the  sye- 
nites or  with  the  porphyritic  felstones  and  granites.  If  the 
absence  of  free  silica  in  the  matrix  be  proved,  it  is  evident 
that  the  affinities  of  minette  are  closer  to  syenite  than  to 
granite,  but  minette  occurs  in  veins  and  dykes  in  both  of 
these  rocks,  and  dykes  of  it  are  also  met  with  in  sedimentary 
deposits  of  Silurian  and  Devonian  age.  It  seems  in  many 
cases  that  micaceous  felstones  approximate  rather  closely  to 
minette.  Kengott  appears  to  entertain  some  such  idea  in 
his  '  Elemente  der  Petrographie.'  V.  Cotta's  definition  of 
minette  is  '  a  felsitic  matrix  containing  much  mica  and 
sometimes  distinct  crystals  of  orthoclase  or  hornblende.' 
The  true  difficulty  seems  to  lie  in  the  imperfect  knowledge 
which  we  as  yet  possess  of  what  a  felsitic  matrix  really  is. 
If  quartz  be  excluded  from  such  a  matrix,  and  it  is  generally 
stated  that  minette  seldom  contains  that  mineral,  then 
1  Mik.  Phys.  d.  Mass.  Gcst.,  p.  122. 


22O  Descriptive  Petrology. 

minette  is  a  mica-syenite  with  a  micro-granular  or  micro- 
crystalline  matrix  ;  if,  on  the  other  hand,  quartz  be  present, 
minette  may  be  closely  allied  to  the  felstones,  micaceous 
felstone  forming  a  transitional  link.  It  is  possible  that  both 
conditions  occur,  and,  if  so,  it  may  become  necessary  to  clas- 
sify the  minettes.  In  seme  of  the  mica-traps  of  the  English 
Lake  District  the  author  has  found  both  orthoclastic  and 
plagioclastic  felspars  which,  in  addition  to  the  magnesian  mica, 
occur  in  well-marked  crystals.  In  such  cases  the  rock  appears 
to  hold  a  position  intermediate  between  minette  and  kersantite. 

If  minette  represent  a  condition  of  the  syenites  which 
are  rich  in  orthoclase,  then  kersantite  is  allied  to  those  which 
are  rich  in  plagioclastic  felspars,  and,  in  such  instances,  it 
may  be  questioned  whether  the  affinities  of  kersantite  are 
not  more  in  the  direction  of  diorite,  especially  of  the  mica- 
ceous varieties  of  that  rock. 

Speaking  approximately,  minette  is  a  rock  which  contains 
magnesian-mica  and  sometimes  hornblende  crystals  in  a 
micro-granular  or  micro-crystalline  matrix  in  which  felspar 
crystals,  mostly  orthoclase,  are  porphyritically  developed, 
while  kersantite  is  a  somewhat  similar  rock  in  which  the 
felspathic  components  are  mainly  plagioclastic. 

Both  minette  and  kersantite  occur,  as  a  rule,  in  the  form 
of  dykes. 

B.  von  Cotta  in  his  remarks  on  syenite  says  :  '  Properly 
speaking  there  are  no  varieties  of  composition  to  adduce, 
unless  we  consider  as  such  those  transitions  into  granite 
and  diorite  which  are  occasioned  by  the  occurrence  of  mica, 
quartz,  and  oligoclase/  l 

This  seems  a  very  just  generalisation,  implying  a  sharp 
definition  of  syenite.  If  the  term  be  allowed  the  wide  scope 
which  some  petrologists  now  accord  to  it,  we  might  as  well 
term  basalt  a  mica-diorite.  That  instances  may  be  found  of 
rocks,  which,  in  mineral  constitution,  form  connecting  links 

1  Rocks  Classified  and  Described^,  by  B.  von  Cotta.  English  transla- 
tion by  P.  H.  Lawrence,  1866,  p.  179, 


Trachyte.  221 

between  very  many,  if  not  all,  of  the  eruptive  rocks  there  is 
no  doubt,  but  sharp,  or  moderate!}  sharp,  definitions  con- 
stitute the  basis  of  all  classification,  and,  if  these  be  aban- 
doned, petrological  nomenclature,  as  it  exists  at  present, 
becomes  almost  worthless. 

A  good  account  of  the  minettes,  kersantite,  and  kersan- 
ton,  by  Delesse,  will  be  found  in  vol.  x.  of  the  '  Annales  des 
Mines,'  for  1857. 

TRACHYTE  GEOUP. 

Trachyte. — (The  rocks  which  have  been  included  under 
this  name  are  exceedingly  numerousytind  the  term  in  its 
present  acceptation  has  still  a  very  wide  range.  The  usual 
constituents  of  trachyte  are  sanidine,  oligoclase,  hornblende, 
sometimes  augite,  magnesian  mica,  magnetite,  titaniferous- 
iron,  tridymite,  and  at  times  some  other  minerals,  such  as 
sodalite,  hauyne,  nosean,  sphene,  mellilite,  leucite,  and  oli- 
vine,  which  may,  for  the  most  part,  be  regarded  as  accessories. 
Plagioclastic  felspar  is  generally  associated  to  a  greater  or 
less  extent  with  the  sanidine  in  these  rocks  ;  hence  Rosen- 
busch,1  Zirkel,2  and  Von  Lasaulx3  consider  that  their  division 
into  sanidine  trachytes  and  sanidine- oligoclase  trachytes 
is  of  little  or  no  account.  The  first  author  suggests  that 
they  may  eventually  be  classified  by  determinations  of  the 
presence  or  absence  of  tridymite,  and  he  thinks  it  probable 
that,  by  noting  the  relative  occurrence  of  hornblende,  augite, 
and  magnesian  mica,  the  trachytes  may  be  arranged  in  a 
series  homotaxial  with  that  into  which  the  syenites  have  been 
divided.  The  sodalite-,  hauyne-,  and  nosean-bearing  trachytes 
appear  to  some  extent  to  be  analogous  to  the  phonolites. 

(The  more  highly-silicated  trachytes  are  comprised  in  the 
group  of  rhyolites,  and,  in  part  at  least,  constitute  the  rhyo- 
lites  proper,  whose  vitreous  condition  is  met  with  in  obsidian^ 
&c.,  as  already  pointed  out.  The  name  trachyte  is  derived 

1  Mik.  Phys.  d.  Massigen  Gesteine,  1877,  p.  200. 
?  Mik.  Besch.  d.  Min.  u.  Gest.  1873,  p.  382. 
1  Elcm.  d.  Petrographie,  1875,  p.  278. 


222  Descriptive  Petrology. 

from  Tpct^vg  (rough),  in  allusion  to  the  rough,  scraping  sen- 
sation which  the  surfaces  of  these  rocks  usually  convey 
when  rubbed  with  the  fingers. 

(Geologically,  the  trachytes  have  been  divided  into 
trachytes  and  trachytic  lavas,  but  the  characters,  even  mi- 
croscopic, of  the  one,  have  not  been  found  to  differ  from 
those  of  the  other.)  The  trachytes  proper  are  mostly  of 
tertiary  or  post-tertiary  age.  Some  rhyolites  are  coeval  with 
them,  while  others  have  a  great  geological  antiquity  ;  but, 
as  yet,  comparatively  little  is  known  of  these  old  rhyolites. 

The  trachytes   may   be   conveniently   classified   in   the 
following  manner  :  — 

Rhyolites  proper    (   i.  Quartz-trachytes        )  allied  to  perlite  and 
or  liparites1          (  ii.  Sanidine-trachytes    )      obsidian. 

"I  allied  to  syenite  and 
the  quartzless  por- 

Trachytes  proper  r11'  Quar'zlfs-trachytes  1-  phyries,   such    as 

P 


1- 
anddormtes  fc,_ 


stone,  &c. 

Analyses  of  these  rocks  show  the  following  approximate 
variations  of  the  amount  of  silica  which  they  respectively 
contain. 

Quartz-trachyte  or  quartz-rhyolite  75  to  77  per  cent,  silica. 

Sanidine-trachyte  or  sanidine-rhyolite    74  to  78       „  „ 

Quartzless  trachyte  or  trachyte  proper   62  to  64       „  „ 

From  this  it  will  be  seen  that,  although  in  some  of  the 
sanidine  trachytes  little  or  no  quartz  can  be  recognised,  even 
microscopically,  yet  they  contain  a  considerably  higher  per- 
centage of  silica  than  the  trachytes  proper. 

It  will  be  well  to  begin,  in  each  case,  with  a  brief  state- 
ment of  the  microscopic  characters  of  the  ground-mass,  base, 
or  matrix  of  each  of  these  three  types,  since  the  megascopic 
appearance  of  these  ground-masses  affords  little  or  no  insight 
as  to  their  mineral  constitution  or  structural  peculiarities  ; 

1  The  term  lithoidite  has  also  been  applied  to  these  rocks. 


v 

Rhyolite  Proper.    ,          v  /  /,  223 

while,  without  a  tolerably  precise  knowledge  lof  tjiese  cha^ac-/ 
ters,  it  is  often  difficult  to  discriminate  correctly  beiw^en  the    /^ 
different  types. 

-//ry.    '';- 

QUARTZ-TRACHYTE  (Quartz-rhyolite,  Liparite):/  >  >  v 

The  matrix  is  generally  micro-aphanitic,  and  contains^/ 
moderate-sized  grains  of  quartz.  The  red  varieties  contain 
more  or  less  peroxide  of  iron,  in  a  finely-divided  state,  or 
in  thin  films.  A  micro-crystalline-granular  or  micro-granitic 
condition  is  less  common  in  the  matrices  of  quartz-trachytes 
than  in  those  of  quartz-porphyries,  but  nevertheless  it  is 
often  present.  The  matrix  of  quartz-trachytes  appears,  to  the 
naked  eye,  as  a  compact,  or  very  finely -granular,  substance, 
often  rough  and  porous ;  it  sometimes  resembles  hornstone, 
or  porcellanite,  while,  at  others,  it  has  a  dull,  earthy  or  kao- 
linised  appearance.  It  varies  considerably  in  colour,  brick- 
red,  reddish-grey  and  yellowish-  and  brownish-white  being 
some  of  the  most  common.  The  principal  bodies  porphy- 
ritically  developed  in  this  matrix  are  crystals  and  crystalline 
granules  of  sanidine  and  quartz,  while  plagioclastic  felspars, 
hornblende,  and  magnesian  mica  (biotite)  are  often  also  well 
developed. 

The  sanidine  crystals  in  the  quartz-trachytes  very  fre- 
quently show  twinning  on  the  Carlsbad  type.  These  crystals 
often  appear  much  fissured  and  fractured.  Under  the 
microscope  they  frequently  exhibit  a  zoned  structure  indica- 
tive of  successive  stages  of  accretion,  and  they  often  show 
numerous  inclosures  of  glass,  gas  and  steam  pores,  well 
developed  crystals  of  quartz,  and  microliths  of  various  kinds. 
Plagioclastic  felspars,  although  frequently  present,  occur, 
as  a  rule,  only  very  sparsely  in  the  quartz-rhyolites,  and  are 
often  so  altered  that  the  characteristic  twin  lamellae  are 
scarcely,  if  at  all,  perceptible,  since  they  are  sometimes 
completely  converted  into  kaolin. 

The  quartz  occurs  both  in  roundish  grains  and  in  definite 
crystals.  These  contain  inclosures  of  glass,  which  are  often 


224  Descriptive  Petrology. 

bounded  by  planes,  corresponding  to  those  of  di-hexahedral 
crystals  of  quartz.  Fluid  lacunae  are  not  yet  known  to  occur 
in  the  quartz  of  quartz-rhyolites,  except  in  one  instance  (in 
the  island  of  Ponza  1). 

This  general  absence  of  fluid  lacunae  distinguishes  the 
quartz  of  these  rocks  from  that  of  granite  in  which  fluid 
inclosures  are  so  common. 

A  little  biotite  frequently  occurs  in  the  quartz-trachytes. 
Hornblende  is  seldom  plentiful.  Tridymite  and  garnets  are 
occasionally  met  with,  but  neither  of  these  minerals  are 
common  accessories.  Magnetite  is  generally  present,  but 
only  in  small  quantity.  Hard,  vesicular  varieties  of  quartz- 
trachyte  occur  in  some  localities,  and  are  known  by  the 
name  millstone-porphyry.  The  vesicles  are  often  lined  with 
chalcedony  or  quartz.  Nodules  or  balls  of  chalcedony  and 
opal  are  met  with  in  the  Hungarian  rocks.  Some  of  the 
quartz-trachytes  show  a  fissile,  slaty,  or  slabby  structure, 
which  sometimes  originates  in  the  varying  character  of 
different  bands  which  exist  in  the  rock,  or  else  in  a  parallel 
arrangement  of  the  sanidine  crystals. 

SANIDINE-TRACHYTE  (Sanidine-rhyolite). 

The  matrix  of  this  rock  is  usually  of  an  aphanitic  or 
micro-crystalline  character.  Under  the  microscope  it  is  seen 
to  consist  almost  wholly  of  little  felspar  crystals,  mostly 
orthoclastic,  but  among  which  plagioclastic  felspars  are 
seldom  absent.  The  felspar  crystals  are  usually  interspersed 
either  with  glass  or  micro-felsitic  matter.  Occasionally,  as 
in  the  sanidine-trachyte  of  Berkum  near  Bonn  on  the  Rhine, 
the  ground-mass  consists  almost  exclusively  of  minute 
sanidine  crystals  mingled  with  microliths  and  granules  of 
hornblende,  and  containing  some  grains  or  interstitial  patches 
of  glassy  matter  and  magnetite.  In  this  matrix  no  quartz 
is  to  be  recognised,  although  the  rock  contains  over  72  per 

1  « On  the  Microscopical  Structure  of  Crystals,  &c.,'  by  II.  C.  Sorby, 
Quart.  Journ.  Geol.  Soc.  Lond.  vol.  xiv.  p.  484. 


Trachyte  Proper.     ,  225 

cent,  of  silica.  Sometimes  the  matrix  of  sanidine-rhyolites 
seems  almost  wholly  amorphous,  or  shows  a  finely-fibrous 
structure,  often  hazy  in  appearance,  while  it  occasionally 
assumes  a  radiate  arrangement  around  certain  points,  thus 
giving  rise  to  spherulitic  structure.  The  spherulites  in  a 
rock  of  this  character  at  Tolcsva,  near  Tokay,  attain  from 
one  to  two  inches  diameter.  Well- individualised  quartz 
sometimes  occurs  in  the  matrix  of  sanidine-trachyte,  some- 
times none  is  visible. 

The  minerals  which  are  porphyritically  developed  in 
these  rocks  are,  for  the  most  part,  crystals  of  sanidine,  either 
single,  or  twinned  on  the  Carlsbad  type,  crystals  of  plagio- 
clastic  felspar,  which,  as  a  rule,  show  more  decomposition 
than  the  sanidine  crystals,  magnesian  mica,  magnetite,  and 
occasionally  hornblende,  tridymite,  and  sphene. 

TRACHYTE  PROPER  (Quartzless  trachyte,  quartzless  sanidine- 
porphyry,  domite). 

(^The  matrix  of  true  trachytes  consists  generally  of  an 
aggregate  of  colourless  felspar  microliths^  which,  by  their 
arrangement  in  certain  directions,  frequently  indicate  fluxion. 
Spiculae  and  granules  of  greenish  hornblende  and  specks  of 
magnetite  are  also,  as  a  rule,  plentifully  mixed  up  with  the 
felspar  microliths.  By  rotation  of  the  section  on  the  stage 
of  the  microscope,  a  very  small  quantity  of  interstitial  glass 
may  usually  be  detected,  by  its  persistent  darkness  between 
crossed  Nicols.(The  general  colour  of  the  matrix  of  trachyte 
is  very  variable,  but  greyish,  yellowish,  and  reddish-brown 
tints  are  the  most  common) 

(  The  larger  porphyritic  crystals  which  occur  in  trachytes 
are  sanidine  and  sometimes  plagioclastic  felspars : l  the 
latter  are  not,  however,  always  present.  Hornblende  is  com- 
mon in  these  rocks)  and  magnesian  mica  also  frequently 
occurs  in  small  crystals  or  scales,  which  are  visible  to  the 

1  The  true  sanidine-trachytes  contain  but  very  little  plagioclastic 
felspars,  and,  in  some  instances,  none. 


226  Descriptive  Petrology. 

naked  eye.  The  sanidine  crystals  are  usually  traversed  by 
numerous  irregular  fissures,  along  which  they  are  often  dis- 
placed or  faulted,  as  though  they  had  been  subjected  to 
strain  and  pressure.  They  are  very  commonly  twinned  on  the 
Carlsbad  type,  and  the  same  may  be  said  of  the  very  minute 
crystals  of  this  mineral  which  occur  so  plentifully  in  the  matrix. 

The  larger  sanidine  crystals  are  sometimes  an  inch  or 
two  in  diameter,  as  in  the  well-known  trachyte  of  the 
Drachenfels,  in  the  Siebengebirge  on  the  Rhine. 

The  triclinic  felspars  are,  as  a  rule,  developed  only  on  a 
small  scale  ;  and,  as  observed  by  v.  Lasaulx,  their  glassy  and 
cracked  appearance  often  renders  it  difficult  to  distinguish 
between  them  and  the  smaller  sanidine  crystals. 

Small  crystals  and  spiculae  of  hornblende  are  common 
in  many  trachytes.  To  the  naked  eye  they  look  black,  or 
greenish-black ;  while,  when  seen  in  thin  sections  by  trans- 
mitted light,  they  appear  green  or  brown. 

(Magnetite  and  apatite  are  also  present,  as  a  rule,  in 
considerable  quantity,  the  former  mineral  frequently  forming 
black,  granular  envelopes  around  the  hornblende  crystal^) 

Tridymite  sometimes  occurs  as  a  constituent  of  the 
matrix,  and  also  in  small  cavities  and  druses  in  the  rock. 
Sphene,  hauyne,  sodalite,  nepheline,  and  specular  iron  are 
not  of  uncommon  occurrence,  while  augite  is  sometimes, 
but  rarely,  met  with  in  trachytes.  In  some  of  the  ejected 
trachytic  blocks,  as  in  those  of  the  Laacher  See,  a  great 
number  of  mineral  species  occur,  including,  besides  those  al- 
ready mentioned,  zircon,  corundum,  meionite,  garnet,  spinel, 
staurolite,  nosean,  olivine,  leucite,  and  various  zeolites. 

''The  name  domite  (from  the  Pny  de  Dome,  in  Auvergne) 
has  been  applied  to  trachytes  which  contain  a  high  percentage 
of  silica,  in  some  instances  over  68  per  cent!  considered  to 
be  due  to  the  presence  of  tridymite,  since(^quartz  is  never 
observed  in  these  rocksj)  while  the  former  'mineral  occurs 
rather  plentifully  in  the  granular-microlitic  matrix,  which 
sometimes  contains  a  small  quantity  of  vitreous  matter. 


Scheme  of  Deviations  from  Trachyte  as  a  Type.    227 

PLATE  IV. 


FELSPAR 

(Mostly  Sanidine) 


LEUCITE 


Qvarfo-lrachi/tc. 
orRhyolite  proper 


AUCITE 


228  Descriptive  Petrology. 

^  In  mineral  constitution  the  domites  do  not  materially 
differ  from  ordinary  trachytes^  their  somewhat  higher 
percentage  of  silica  being  their  chief  characteristic,  as 
pointed  out  by  Zirkel  and  Rosenbusch.  (The  porphyritic 
crystals  in  them,  however,  seldom  attain  any  great  size.) 

/The  domites  form  some  of  the  most  conspicuous  dome- 
shaped  hills  or  '  puys '  which  constitute  such  striking 
features  in  the  scenery  of  Auvergne) — (Vide  Scrope's 
'  Volcanos  of  Central  France.') 

fTrachytic  conglomerates  and  tuffs  are  composed  of 
fragments  of  trachytic  rocks,  together  with  fragments  of  other 
eruptive  and  sedimentary  rocks);  these  are  frequently  rounded, 
and  cemented  by  crumbling  earthy  matter,  mostly  derived 
from  the  fine  detritus  resulting  from  the  disintegration  of 
trachytes.  VjThe  tuffs  are  either  of  an  earthy  or  granular  and 
sandy  character,  mostly  light-coloured — pale  buff,  grey,  or 
yellowish-white.  They  contain  fewer  rock-fragments  than 
the  conglomerates,  but  pass  into  the  latter  as  the  fragments 
become  more  numerous^  They  generally  contain  crystals  of 
sanidine,  biotite,  and  other  mineral  components  of  trachyte. 
It  seems  difficult  or  impossible  to  draw  any  hard  line  in 
the  classification  of  these  rocks.  Sometimes  they  appear  to 
have  resulted  simply  from  the  weathering  of  trachytes,  at 
others  they  have  more  the  character  of  volcanic  ejectamenta, 
ashes,  &c.,  which  have  been  deposited  in  water  ;  and,  in 
both  cases,  there  is  frequently  an  admixture,  to  a  greater  or 
less  extent,  of  detrital  matter  derived  from  sedimentary 
rocks. 

PHONOLITE  GROUP. 
/ 

(The  rocks  termed  phonolite  or  clinkstone  are  in  a 
certain  degree  related  to  the  trachytes  proper.;  The  name 
phonolite,  from  ^ovot,  sound,  was  first  given  to  them  by 
Klaproth.  (JBoth  this  and  the  other  name,  clinkstone,  bear 
reference  to  the  ringing  or  clinking  sound  which  slabs  or 
thin  fragments  emit  when  struck  with  the  hammer} 

(The   constituent   minerals   of   phonolite   are   sanidine, 


Phono  lite.  229 

nepheline,  and  generally  more  or  less  hornblende  and 
magnetite.^)  Nosean  and  hauyne  are  often  present  in  con- 
siderable quantity,  also  leucite,  and  sometimes  tridymite. 
The  minerals  which  are  less  common,  and  less  important  as 
constituents  of  phonolite,  are  augite,  olivine,  sphene,  zircon, 
apatite,  and  titaniferous  iron.  Oligoclase  as  well  as  sanidine 
occurs  in  some  of  these  rocks,  and  these  oligoclase-sanidine 
phonolites  are  so  closely  related  to  the  trachytes  that  they 
have  received  the  name  of  trachy-phonolite. ! 

\The  matrix  or  ground-mass  of  phonolite  is  micro- 
crystalline,  and  presents  either  a  rough  and  porous,  or  a 
compact,  character.  The  colour  is  usually  grey,  of  a 
yellowish  or  slightly  greenish  tint.  It  is  partly  soluble  in 
hydrochloric  acid)  the  soluble  portion  being  represented 
by  nepheline  and  zeolitic  decomposition  products  of  that 
mineral,  while  the  felspathic  portion  of  the  matrix  constitutes 
the  insoluble  part. 

([The  smaller  the  percentage  of  the  insoluble  matter,  the 
higher,  as  a  rule,  is  the  percentage  of  water  which  the  rock 
contains,  and  this  is  usually  accompanied  by  an  increase 
in  its  specific  gravity.  The  larger  the  percentage  of  silica 
which  a  phonolite  contains,  the  less,  as  a  rule,  is  its  percentage 
of  soluble  material.)  The  amount  of  silica  in  phonolites 
generally  ranges  fro'm  50  to  a  little  over  60  per  cent. 

(Jhe  phonolites  fuse  easily,  before  the  blowpipe,  to  a 
whitish  or  greenish  glass,  and  yield  more  or  less  water  when 
heated.) 

The  phonolites  may  be  divided,  according  to  their 
dominant  mineral  constituents,  into  the  following  sub- 
groups : — 

Nepheline-phonolite,   Hauyne-phonolite,   Nosean-phonolite,  and 
Felspar-phono  lite. 

1  A  very  complete  account  of  the  different  varieties  of  phonolite  is 
given  in  Boricky's  Petrographische  Studien  an  den  Phonolithgesteinen 
Bohmens  (Archiv  d.  Naturw.  Landesdurchforschimg  v.  Bbhmen.  Band 
iii.  Geql.  Abth.)  Prag,  1873. 


230  Descriptive  Petrology. 

The  phonolites  have,  however,  also  been  classified  by 
Dr.  Emanuel  Boficky  in  the  following  manner  : — 

i.  Nepheline-phonolite. — With  a  compact  matrix,  and 
containing  much  nepheline,  and  porphyritic  crystals 
of  sanidine. 

ii.  Leiicite-nepheline-phonolite. — With  a  matrix  of  leucite 
and  nepheline,  much  pyroxene,  amphibole,  and 
magnetite,  but  with  sanidine  poorly  represented. 

iii.  Nepheline-nosean-phonolite  (Nepheline-hauyne-pho- 
nolite). — Containing  much  nosean  and  some 
hauyne,  with  a  little  sanidine,  pyroxene,  amphi- 
bole, titaniferous-iron,  and  sphene. 

iv.  Leu cite-nosean-phono lite  (Leucite-hauyne-phonolite). 
— Consisting  mainly  of  leucite,  together  with  some 
nosean  or  hauyne,  and  more  or  less  nepheline  and 
sanidine.  The  leucite  occurs  both  of  microscopic 
,and  megascopic  dimensions. 

v.  Sanidine-nosean-phonolite  (Sanidine-hauyne-phono- 
lite). — A  light-coloured  rock,  speckled  with  nosean 
and  with  a  variable  amount  of  porphyritic  sanidine. 

vi.  Nepheline-sanidine-phonolite. — A  greenish,  yellowish- 
grey,  or  dark  grey,  slaty  or  compact  rock,  weather- 
ing greyish-white,  containing  numerous  porphyritic 
crystals  of  sanidine,  and  a  few  of  augite  or  horn- 
blende. 

vii.  Oligodase-sanidine-phonolite  (Trachy-phonolite). — 
Containing  from  about  5  to  30  per  cent,  of  triclinic 
felspar. 

viii.  Sanidine-phonolite. — Sanidine  is  abundant.  Nephe- 
line and  nosean  occur  in  variable  quantity  up  to 
30  per  cent.,  the  former  mineral  constituting  a 
large  proportion  of  the  matrix.  Sanidine,  augite, 
and  hornblende  occur  porphyritically ;  also  occa- 
sionally a  little  mica  and  sphene. 

The  relative  proportions  of  the  different  minerals  which 
constitute  the  matrix  of  phonolite  vary  considerably,  the 
soluble  portion  varying  in  its  relation  to  the  insoluble 


Phonolite.  231 

portion  from   15  to  55  per  cent,  of  the  rock,  according  to 
A.  von  Lasaulx. 

The  microscopic  character  of  the  matrix  is  generally 
micro-crystalline,  and,  in  the  phonolites  of  some  localities, 
consists  almost  exclusively  of  superposed  layers  of  small, 
well-defined  crystals  of  nepheline  and  sanidine,  with  little, 
sparsely  scattered  crystals  of  hornblende  and  magnetite. 

No  amorphous  or  micro-aphanitic  substance  occurs  in 
the  matrix  of  phonolite  ;  but  its  micro-crystalline  nature  is 
not  always  clearly  perceptible,  owing  to  the  transparent  and 
colourless  character  which  it  often  exhibits. 

The  sanidine  is  very  commonly  twinned  on  the  Carlsbad 
type,  and  is  frequently  more  or  less  altered.  Minute  crystals 
of  nepheline,  nosean,  and  hornblende,  and  granules  of 
magnetite,  are  sometimes  seen  lying  within  the  crystals  of 
sanidine,  and  often  appear  closely  ranged  along  the  margins 
of  the  sections  of  these  crystals,  which  also,  at  times,  contain 
inclosures  of  glass. 

Plagioclastic  felspars  are  only  of  exceptional  occurrence 
in  phonolites. 

The  nepheline  crystals,  although  they  occasionally  attain 
moderate  dimensions,  are,  as  a  rule,  very  minute,  especially 
those  which  enter  into  the  constitution  of  the  matrix.  They 
show  sharply  denned  boundaries,  and  in  some  phonolites 
are  very  numerous  ;  while  in  others,  the  mineral  is  so  poorly 
represented,  that  the  rocks  approximate  to  trachytes.  The 
nepheline  crystals  frequently  show  signs  of  alteration  which, 
in  its  ultimate  phase,  results  in  the  development  of  zeolitic 
matter,  probably  natrolite.  Other  zeolites,  such  as  stilbite, 
thomsonite,  chabasite,  analcime,  apophyllite,  &c.,  also  occur 
in  phonolites. 

The  decomposition  commences  by  the  development  of 
a  yellowish  fringe,  which  gradually  passes  from  the  exterior 
to  the  interior  of  the  crystal,  and,  until  these  fringes  unite,  a 
nucleus  of  unaltered  nepheline  remains. 

Hauyne   is   often  very    plentiful,    and    there   are    few 


232  Descriptive  Petrology. 

phonolites  in  which  it  is  totally  absent,  except  in  those  rocks 
in  which  leucite  takes  the  place  of  nepheline.  Hauyne  and 
nosean  are  so  closely  related,  both  chemically  and  mor- 
phologically, that  some  mineralogists  regard  them  as  one 
species,  and  Rosenbusch  includes  them  both  under  the 
older  name  hauyne.  After  treating  a  section  containing 
these  minerals  with  a  drop  of  hydrochloric  acid,  it  is  possible 
to  distinguish  the  sulphate- of-lime-bearing  hauyne  from  the 
sulphate-of-soda-bearing  nosean,  by  examining  the  section 
under  the  microscope,  since,  after  a  little  time,  the  decom- 
position of  the  hauyne  gives  rise  to  little  needles  of  gypsum, 
frequently  associated,  if  a  gentle  heat  be  previously  applied, 
with  little  rhombic,  cube-like,  doubly-refracting  crystals  of 
anhydrite.1  These  minerals  vary  considerably  in  colour, 
appearing  brown,  blue,  yellow,  green,  black,  and  colourless. 
Some  observers  consider  that,  in  their  normal  condition,  they 
are  colourless,  and  that,  at  all  events,  some  of  the  colours 
are  due  to  changes  engendered  by  an  elevated  temperature, 
since  colourless  hauyne  may  be  artificially  coloured  by 
heat.  The  decomposition  of  these  minerals  gives  rise  to 
the  development  of  zeolitic  matter,  and  also  [when,  as  in 
the  case  of  hauyne,  they  contain  a  fair  amount  of  sulphate 
of  lime]  calcspar  is  formed. 

Both  hornblende  and  augite  occur  in  some  phonolites, 
and  it  is  often  very  difficult  to  distinguish  the  one  mineral 
from  the  other,  since  the  augite  in  some  cases  exhibits 
strong  dichroism,  while  in  hornblende  this  character  is 
sometimes  quite  absent.  In  such  cases  the  angles  of  inter- 
section of  the  cleavage  planes,  when  they  can  be  observed, 
afford  a  much  safer  means  of  discrimination  than  the 
phochroic  characters  of  these  minerals. 

Magnetite  is  nearly "  always,  and  titaniferous-iron  is 
occasionally,  present  in  phonolites.  Biotite,  leucite,  and 
tridymite  are  also  often  present  in  moderate  quantity ; 
while  olivine,  apatite,  garnet,  and  zircon  are  among  the  less 

1  Mik.  Phys.  d.  Massi»en  Gesteine.     Rosenbusch,  1877,  p.  218. 


Phonolite.  233 

frequently  occurring  constituents.  The  phonolites  which 
decompose  most  readily  are,  as  a  rule,  those  which  are 
richest  in  nosean. 

^Phonolite  occurs  occasionally  in  the  form  of  lava  flows 
but  more  commonly  in  conical  masses  or  hills.  It  some- 
times exhibits  well-marked  columnar  structure,  and  has  a 
very  general  tendency  to  split  into  slabs  or  slates,  the  more 
finely-cleavable  varieties  being  used  for  roofing  purposes  in 
certain  localities)  In  advanced  stages  of  weathering  the 
rock  passes  into  an  earthy  condition,  known  as  phonolite- 
wacke. 

Phonolite-conglomerate. — In  some  stages  of  disintegration 
phonolite-conglomerates  are  also  formed  ;  these  consist  of 
fragments  of  phonolite,  and  often  of  other  rocks,  together 
with  fine,  disintegrated  phonolitic  matter  ;  the  whole  being 
frequently  bound  together  by  a  calcareous  cement.  These 
conglomerates  are  mostly  found  at  the  bottoms  of  the 
phonolite  hills,  from  which  their  materials  have  been  de- 
rived. 

Phonolite-tiiff  is  an  earthy  rock  of  somewhat  similar 
character,  except  that  it  contains  but  few  actual  rock- 
fragments.  This  earthy  phonolitic  matter  often  contains 
numerous  crystals  of  the  constituent  minerals  of  phonolite, 
and  the  rock  is  generally  cemented  by  more  or  less  carbonate 
of  lime.  The  eutaxites  of  the  Canary  Islands,  and  the 
piperno  of  Pianura,  near  Naples,  are  agglomeratic  and 
banded  lavas,  which  are  considered  to  be  more  or  less 
closely  related  to  phonolite.  The  former  have  a  partly 
vitreous  character,  and  contain  rock-fragments  lying  in 
tolerably  regular  layers,  which  impart  a  flecked  or  banded 
appearance  to  the  lava,  into  which  the  fragments  are  partially 
fused.  Want  of  space  precludes  any  detailed  account  of 
these  rocks,  but  descriptions  will  be  found  in  the  '  Geolog- 
ische  Beschreibung  der  Insel  Teneriffe,'  Fritsch  u.  Reiss. 
Winterthur,  1868  ;  and  in  the  works  of  Rosenbusch  and 
v.  Lasaulx,  already  cited. 


234  Descriptive  Petrology. 

ANDESITE  GROUP. 

(The  name  andesite  was  first  used  by  L.  von  Buch  for 
certain  rocks  occurring  in  the  Andes.  The  felspar  in  these 
rocks  is  plagioclastic,  and  is  referred  sometimes  to  andesine 
and  sometimes  to  oligoclase.  The  other  principal  consti- 
tuents are  hornblende,  augite  and  quartz,  while  more  or  less 
magnetite  is  also  present  as  a  rule.  These  rocks  were  first 
divided  by  Roth  into  hornblende-andesites  and  augite-ande- 
sites.  The  former  are  closely  related  to  the  trachytes,  the 
latter  to  the  basalts,  and  they  thus  constitute  a  connecting 
link  between  these  highly  basic  and  highly  silicated  rocksf) 
a  post  also  occupied  to  some  extent,  although  upon  different 
mineralogical  grounds,  by  the  trachy-dolerites. 

(in  the  rocks  of  both  divisions  of  the  andesite  group 
quartz  is  sometimes  present,  sometimes  absent)  and,  upon  the 
presence  or  absence  of  this  mineral,  the  aridesites  may  be 
glassed  as 

Hornblende-andesite  f  Quartzose  hornblende-andesite  or  dacite.' 
( Quartzless  hornblende-andesite. 
t  Quartzless  augite-andesite. 

Augite-andesite          \  Quartzose    augite-andesite    (of   doubtful 
^     authenticity). 

Diallage-  and  hypersthene-andesites  have  also  been  de- 
scribed by  Drasche. 

(Quartzose  Datite  consists  of  a  finely-granular  or  compact, 
grey,  brownish  or  greenish-grey  matrix,  containing  crystals 
of  plagioclase  (oligoclase  or  andesine)  and  sanidine,  spiculse 
of  hornblende,  and  granules  and  crystals  of  quartz.  Under 
the  microscope,  the  matrix  is  seen  to  consist  of  microliths  of 
plagioclase,  sanidine  and  hornblende,  together  with  fine 
grains  of  magnetite.  Quartz  seldom  appears,  according  to 
A.  von  Lasaulx,  to  enter  into  the  composition  of  the  matrix) 
when  definite  megascopic  grains  of  quartz  are  visible  in  the 
lock. 

f As  a  rule,  the  matrix  is  entirely  micro-crystalline,  but  at 
(  So  named  from  its  extensive  occurrence  in  DaciaA 


.Andesite.  235 

times,  when  examined  microscopically,  it   shows  here  and 
there  a  very  small  quantity  of  interstitial  glass} 

•(The  triclinic  felspars  are  the  most  numerous  and  impor- 
tant of  the  porphyritic  constituents  of  this  rock,  and  analyses 
indicate  that  they  may  sometimes  be  referred  not  only  to 
andesine  and  oligoclase,  but  also  to  labradoriteA  Crystals, 
showing  interlamellation  of  triclinic  felspar  with  sanidine,  are 
sometimes  to  be  seen  under  the  microscope. 

The  quartz  usually  contains  fluid  lacunae  and  magnetic 
dust. 

The  hornblende  is  either  in  spiculae,  or  in  well-developed 
little  crystals,  which  sometimes  show  twinning,  and  seldom 
occurs  in  forms  of  purely  microscopic  dimensions.  It  shows 
strong  dichroism,  and  often  contains  needles  of  apatite'  and 
grains  of  magnetite.  Epidote  and  chlorite  represent  the 
ultimate  phase  of  alteration  of  the  hornblende.  Occa- 
sionally crystals  of  augite  may  be  detected  in  these  horn- 
blend  e-andesites.  Olivine  is  never  met  with  in  them. 
Some  of  the  hornblende-andesites  of  Hungary  may  be 
regarded  as  rhyolites,  in  which  plagioclastic  felspars  play  the 
part  of  sanidine. 

The  quartzose  dacites  have  been  divided  into  trachytic 
dacites,  biotite  dacites,  &c.;  in  the  latter  hornblende  is  almost 
entirely  absent,  its  place  being  represented  by  biotite.  Some 
of  these  rocks  are  very  poor  in  quartz,  and  they  then  pass 
into  the  quartzless  hornblende-andesites. 

(The  chemical  composition  of  the  dacites  varies  consider- 
ably in  the  amount  of  silica  which  is  present,  this  fluctuation 
being  due  to  the  variable  quantity  of  quartz  which  different 
dacites  contain^)  Von  Lasaulx  gives  as  a  mean  analysis  :  — 


SiO2=66-io.Al2O3=  14-80.  FeO=6'3o.  CaO=5'3o.  MgO 
=  2-40.  K2O  and  Na2O=7'7o.  H2O=--o'5o.  <Jn  unaltered 
samples  of  dacite  the  soda  is  always  in  excess  of  the  potash,^) 
Quartzless  Hornblende-Andesite.  —  These  rocks  mainly 
differ  from  the  preceding  in  containing  no  quartz  and  little 
or  no  sanidine.  Biotite  and  magnetite  are  plentiful  in  them. 


236  Descriptive  Petrology. 

Augite  sometimes   occurs,   also   nephelme.      Hauyne   and 
olivine  are  very  rarely  met  with, 

Quartzless  hornblende- andesites  are  well  represented  in 
the  Auvergne,  the  Siebengebirge  and  other  localities,  where 
they  have  commonly  been  designated  trachytes. 

A.  von  Lasaulx  gives  the  following  as  a  mean  analysis  of 
these  rocks — 

SiO2  =  5975  .  A12O3  =  17-25  .  FeO  and  Fe2O3  =  7-57 .  CaO  =  6 . 
MgO  =  i'3o.  K2O  =  3'io.Na2O  =4.  H2O  =  i. 

Augite- Andesites. —  The  rocks  which  come  under  this 
denomination  are  closely  related  to  basalt  in  their  minera- 
logical  constitution.  They  have  a  compact  or  finely-crys- 
talline matrix,  containing  a  considerable  quantity  of  glass. 
The  constituent  minerals  are  triclinic  felspar  (either  oligo- 
clase  or  andesine),  augite,  magnetite,  and  at  times  more  or 
less  sanidine  and  hornblende,  while  occasionally  quartz  may 
be  present.  These  rocks  have  been  divided  into  quartz- 
less-  and  quartzose-augite-andesites,  but  the  latter  appear  to 
be  of  very  exceptional  occurrence,  and  it  is  probable  that 
they  approximate  more,  in  some  instances,  to  the  hornblende- 
andesites. 

The  quartzless-augite-andesites  frequently  exhibit  very 
distinct  fluxion  structure.  The  matrix  is  usually  of  a  brown- 
grey  or  brackish  colour.  The  glass  which  enters  into  the 
constitution  of  the  matrix  is  sometimes  quite  clear  and  often 
contains  trichites,  at  other  times  the  glass  is  devitrified  by 
the  development  of  granular  structure.  Within  this  matrix 
crystals  of  oligoclase  or  andesine,  augite  and  magnetite,  are 
developed,  and  crystals  of  sanidine  and  biotite  also  fre- 
quently occur.  Olivine  and  nosean  are  but  rarely  met  with 
in  this  rock. 

The  felspar  and  augite  crystals  frequently  exhibit  inclo- 
sures  of  glass.  The  amount  of  sanidine  present  is  always 
subordinate  to  that  of  the  plagioclase. 

The  silica  in  the  quartzless  augite- andesites  is  sometimes 
more,  sometimes  less  than  60  per  cent;  the  potash  is  a  little 


Porphyrite.  237 

over  or  under  3  per  cent. ;  and  the  soda  varies  from  a  little 
over  2  to  about  4  per  cent. 

The  augite-andesites  are  essentially  lavas. 

Propylite  is  a  rock  closely  allied  in  mineral  constitution 
to  the  hornblende-andesites.  It  occurs  very  extensively  in 
the  United  States,  and  bears,  geologically,  close  affinities  to 
volcanic  rocks.  Propylites  also  occur  in  Transylvania  and 
Hungary,  where,  as  at  Kapnik  and  Nagybanya,  they  contain 
rich  metalliferous  veins.  Propylites  are  of  early  tertiary  age. 
They  are  in  fact  the  first,  or  oldest,  eruptive  rocks  of  that 
epoch.  The  opinions  of  Stache  and  v.  Richthofen  appear  to 
differ  concerning  this  rock.  Zirkel  adopts  the  views  of  the 
former  author,  and  says,  *  That  petrographical  differences 
exist  between  propylite  and  hornblende-andesite  cannot  be 
any  longer  doubted.' 1  He  describes  some  of  the  typical 
propylites  as  consisting  of  felspars  completely  filled  with 
hornblende  material,  while  the  larger  hornblende  crystals  are 
entirely  altered  into  vivid-yellow  epidote,  slightly  tinged  with 
pale-green,  occurring  either  in  confused,  fascicular,  radiating 
aggregates,  or  in  small  roundish  grains.  Apatite  is  also  pre- 
sent. Zirkel  regards  these  rocks  as  more  closely  allied  to 
porphyritic  diorites  than  to  andesites.  His  explanations, 
however,  still  seem  to  fail  in  giving  a  sharp  definition  for 
these  rocks.  The  propylites  are  both  quartzless  and  quartz- 
ose,  like  the  andesites. 

The  propylites  are  the  equivalents  of  the  greenstone- 
trachytes  of  v.  Richthofen,  while  the  grey -trachytes  of  that 
author  are  represented  by  the  hornblende-andesites. 

Stache  applied  the  term  dacite  to  both  of  these  groups. 
Zirkel  objects  to  this  extended  application  of  dacite,  and 
argues  that  propylite  is  an  independent  rock. 
PORPHYRITE  GROUP. 

The  rocks  comprised  under  the  term  porphyrite  are 
characterised  by  an  aphanitic  or  micro-crystalline  matrix, 
essentially  composed  either  of  triclinic  felspar  and  horn- 

1  Microscopic  Petrology.     U.  S.  Exploration  of  4Oth  Parallel,  p.  10. 


238  Descriptive  Petrology. 

blende,  or  of  triclinic  felspar  and  augite,  in  which  larger 
porphyritic  crystals  of  the  same  minerals  are  developed, 
while,  in  some  instances,  biotite  takes  the  place  of  hornblende. 
From  the  mineral  constitution  of  these  rocks  it  is  therefore 
evident  that  those  consisting  of  triclinic  felspar  and  horn- 
blende are  allied  to  diorite,  while  those  containing  triclinic 
felspar  and  augite  may,  since  the  felspar  is  usually  oligoclase, 
be  regarded  as  approximations  to  diabase.  Those  in  which 
biotite  acts  as  a  substitute  for  hornblende  are  more  or  less 
closely  related  to  the  mica-diorites.  The  porphyrites  are 
therefore  divided  into  diorite  and  diabase-porphyrites. 

These  rocks  may  to  some  extent  be  considered  as  iden- 
tical with  diorite  and  diabase,  differing  from  them  chiefly  in 
the  fact  that  they  have  a  micro-aphanitic,  or  in  some  cases 
a  felsitic  matrix,  while  diorite  and  diabase  are  crystalline  gra- 
nular throughout.  In  the  diorite-porphyrites  quartz  is  some- 
times present,  sometimes  absent :  in  the  diabase-porphyrites 
it  is  of  very  exceptional  occurrence. 1 

Diorite- Porphyrites. — The  matrix  of  these  rocks  is  usually 
dark-brown  or  dark-grey,  and  may  either  be  of  a  felsitic  cha- 
racter, or  it  may  consist  of  an  admixture  of  microliths  of 
oligoclase  and  hornbende.  It  is  in  both  cases  essentially 
crystalline,  or  micro -granular,  never  micro  aphanitic.  Those 
rocks  which  possess  a  matrix  of  the  former  kind  constitute 
the  division  of  quartzose-diorite  -porphyrites,  and  quartz 
occurs  in  them  not  only  microscopically  in  the  felsitic 
matrix,  but  also  frequently  in  grains  of  moderate  size 
which  are  visible  to  the  naked  eye.  Those  diorite-por- 
phyrites which  have  a  microlitic  matrix  and  contain  no 
quartz  constitute  the  division  of  quartzless  diorite-por- 
phyrites. Occasionally,  but  rarely,  the  microscope  shows 
the  presence  of  a  small  quantity  of  vitreous  matter  in 
the  matrix. 

1  It  seems  probable  that  in  some  instances  a  relation  may  exist  be- 
tween some  of  the  porphyrites  and  eurites.  Kinahan,  in  his  Handy- 
Book  of  Rock  Names,  London,  1873,  p.  49,  ascribes  a  similar  opinion 
to  Naumann. 


Porphyrite.  239 

According  to  the  nature  of  the  minerals  which  are  por- 
phyritically  developed  in  them,  the  diorite  porphyrites  may 
be  distinguished  as  plagioclase-porphyrite,  in  which  crystals 
of  oligoclase  are  met  with,  while,  at  times,  granules  of 
quartz  and  crystals  of  biotite  are  also  visible  as  porphyritic 
developments.  Hornblende-porphyrite,  in  which  distinct 
crystals  of  hornblende  and  triclinic  felspar  occur  porphy- 
ritically.  In  some  hornblende-porphyrites,  in  addition  to 
oligoclase,  a  very  little  hornblende  is  occasionally  visible. 
The  mica-porphyrites  are  distinguished  by  the  porphyritic 
development  of  biotite  and  oligoclase,  the  former  mineral 
almost  entirely  supplanting  the  hornblende.  Quartz  is  very 
generally  present  in  this  rock. 

The  rocks  kersantite  and  kersanton  which  occur  in  the 
form  of  dykes  in  certain  parts  of  Brittany  are  closely  related 
to  the  mica-porphyrites.  They  consist  of  biotite  porphyriti- 
cally  developed  in  a  greenish-grey  matrix,  which  consists  in 
great  part  of  oligoclase  and  which  also  at  times  occurs  in 
well-developed  little  crystals.  Kersantite  differs  mainly 
from  kersanton  in  containing  more  or  less  hornblende. 
These  rocks  are  sometimes  amygdaloidal,  and  commonly 
contain  minerals  of  secondary  origin,  such  as  chlorite,  calc- 
spar,  epidote,  pyrites,  &c. 

Diabase- Porphyrites. — These  rocks  have  a  finely  aphanitic 
or  granular  matrix,  consisting  of  triclinic  felspar  and  augite, 
and  the  same  minerals  also  constitute  the  porphyritic  crystals 
which  occur  in  this  matrix.  The  plagioclase  crystals,  which 
predominate  over  those  of  augite,  are  sometimes  labradorite 
and  sometimes  oligoclase.  Quartz  is  not  of  common  occur- 
rence in  these  rocks. 

The  diabase-porphyrites  have  been  divided  into  diabase 
porphyry  and  augite  porphyry :  the  term  <  porphyrite ' 
will,  however,  here  be  substituted  for  '  porphyry/  since 
the  latter  term  implies  the  megascopic  development  of 
certain  minerals,  and,  when  coupled  with  the  prenomen 
*  diabase,'  may  therefore  be  considered  objectionable  by 


240  Descriptive  Petrology. 

those  who  consider  that  it  is  better  in  all  cases  to  use 
the  adjective  ' porphyritic '  and  to  abandon  the  term  'por- 
phyry/ 

Diabase-porphyrite,  or  plagioclase-diabasite,  consists  of 
crystals  of  augite  and  labradorite,  or  oligoclase,  somewhat 
sparsely  disseminated  in  a  dark  grey  or  greenish-grey  matrix, 
which  is  compact  and  aphanitic,  and  consists  of  microliths 
and  little  crystals  of  triclinic  felspars  and  augite,  with  occa- 
sional traces  of  vitreous  matter.  The  felspars  sometimes 
exhibit  a  greenish  appearance,  due  to  an  admixture  with 
chlorite.  The  pyroxenic  constituents  of  the  rock  are  fre- 
quently represented  by  pseudomorphs  of  serpentinous  or 
chloritic  substances  (viridite).  Calcspar  and  epidote  are 
also  among  the  usual  secondary  products.  Magnetite  occurs 
more  or  less  plentifully.  The  verde-antique  porphyry  is  one 
of  the  diabase-porphyrites. 

Augite-porphyrite,  or  augite- diabasite,  has  also  a  compact 
aphanitic  matrix  in  which  porphyritic  crystals  of  triclinic 
felspar  and  augite  are  developed,  but  the  felspar  is  always 
labradorite,  and  the  porphyritic  crystals  of  augite  are,  as  a 
rule,  considerably  in  excess  of  the  felspar  crystals.  The 
matrix,  moreover,  very  frequently  contains  a  certain  amount 
of  glass.  Olivine  crystals  and  pseudomorphs  after  olivine 
are  also  common  in  the  augite-porphyrites. 

Apatite,  pyrites,  chlorite,  and  calcspar  are  frequently 
met  with,  and  occasionally  a  little  orthoclase  is  present. 

The  augite  is  in  some  cases  entirely,  partially  in  others, 
converted  into  hornblendic  matter,  which  is  dichroic,  and 
shows  the  characteristic  hornblende  cleavage.  This  product 
of  the  alteration  of  augite,  which  is  known  as  uralite,  is 
occasionally  well  developed  in  these  rocks,  which  are  thence 
termed  uralite  porphyries. 

The  augite-porphyrites  occur  in  dykes  or  intrusive  sheets, 
and  in  some  instances  they  may  represent  lava  flows. 
Sometimes  they  are  vesicular  and  amygdaloidal.  In  the 
latter  case  the  vesicles  often  contain  many  different  species 


Diortte.  241 

of  zeolites  and  other  minerals  of  secondary  origin.  Tufaceous 
conditions  of  these  rocks  occur  in  the  Tyrol. 

DIORITE  GROUP. 

The  term  '  greenstone,'  which  in  its  older  signification 
embraced  basalt,  diabase,  gabbro,  diorite,  &c.,  has  subse- 
quently been  restricted  in  its  application,  and  employed  as  a 
synonym  for  diorite.  Since,  however,  the  name  greenstone  is 
almost  meaningless,  it  seems  desirable  either  to  discard  it,  or, 
still  better,  to  use  it  in  its  original  sense  as  an  ambiguous  and 
comprehensive  term,  useful  in  field  geology,  but  otherwise  only 
admissible  as  an  expression  of  comparative  ignorance,  such 
as  may  safely  be  employed  in  the  case  of  rocks  of  a  certain 
type,  which  have  reached  so  advanced  a  stage  of  decompo- 
sition, and  in  which  the  constituent  minerals  are  so  poorly 
developed,  that  it  is  no  longer  safe  or  possible  to  hazard  any 
opinion  concerning  their  precise  normal  mineralogical  con- 
stitution. 

Diorite  is  an  essentially  crystalline-granular  admixture 
of  triclinic  felspar  and  hornblende.  The  majority  of  the 
diorites  are  quartzless,  nevertheless  quartz  occurs  in  some  of 
them,  and  they  are  then  designated  quartz-diorites.  The 
triclinic  felspar  is  sometimes  oligoclase,  sometimes  labra- 
dorite,  and  this  fact,  again,  gives  rise  to  a  division  into 
oligoclase-diorites  and  labrador-diorites. 

Oligodase-diorite  is  a  crystalline-granular  admixture  of 
oligoclase  and  hornblende.  The  texture  of  the  rock  varies 
from  fine  to  coarse  grained.  The  colour  also  is  variable, 
being  sometimes  greenish-grey,  at  others  greenish-black, 
while  some  of  the  coarser  grained  varieties  have  a  speckled 
or  blotched  appearance.  The  rocks  are  sometimes  very 
compact  in  texture,  and  are  then  styled  diorite-aphanites, 
but  in  the  most  compact  varieties  it  is  seldom,  even  under 
the  microscope,  that  any  traces  of  a  micro-aphanitic  or  of  a 
devitrified  paste  can  be  detected.  Glass  inclosures  and  gas- 
pores  are  not  of  common  occurrence  in  the  oligoclase 

R 


242  Descriptive  Petrology. 

crystals,  but  the  latter  sometimes  contain  fluid  lacunae.  The 
oligoclase  is  usually  white  or  greenish-white,  and  shows  the 
twinning  striation  characteristic  of  triclinic  felspars.  Ortho- 
clase  is  sometimes  present  in  these  diorites,  but  it  plays  a 
very  subordinate  part  as  compared  with  the  oligoclase.  The 
hornblende  is  mostly  greenish-black,  sometimes  brownish, 
and  occurs  in  long  blade-like  crystals  or  in  imperfectly 
developed  crystals,  and  irregularly  shaped  patches  and 
grains.  In  the  finer  grained  and  aphanitic  varieties  of  the 
rock  the  hornblende  crystals  are  often  exceedingly  small. 
A  microscopic  examination  of  thin  sections  of  diorite  fre- 
quently shows  the  presence  of  numerous  inclosures  of 
glass,  magnetite  dust,  and  microliths,  in  the  hornblende 
crystals,  together  with  gas-pores. 

Crystals  of  magnesian  mica  often  occur  in  these  rocks, 
and,  when  very  numerous,  they  constitute  the  varieties  known 
as  mica-diorites.  Apatite  is  present  in  nearly  all  diorites, 
and  the  little  hexagonal  prisms  of  this  mineral  may  often, 
when  sections  are  examined  under  the  microscope,  be  seen 
to  colonise  in  particular  spots.  Augite  is  occasionally,  but 
not  often,  present.  Chlorite,  pyrites,  magnetite,  and  titani- 
ferous-iron  are  of  very  common  occurrence  in  diorites, 
while  garnet,  sphene,  and  epidote  are  also  met  with  as 
accessory  constituents. 

Labrador-diorite. — If  labradorite  be  substituted  for 
oligoclase,  the  foregoing  description  will  answer  equally  well 
for  this  variety. 

Quartz-diorite. — In  the  constitution  of  these  rocks  quartz 
plays  a  somewhat  important  part,  and  occurs  both  in  mega- 
scopic and  in  microscopic,  roundish  or  angular  grains,  but 
seldom  in  properly-developed  crystals. 

Fluid  lacunae  are  very  numerous  in  some  of  these  quartz 
grains,  and,  in  addition  to  bubbles,  they  occasionally  contain 
minute  cubic  crystals  of  rock-salt,  as  in  the  quartz-diorites 
of  Quenast,  in  Belgium,  described  by  Renard.  (See  fig.  74, 
p.  165.)  A  very  large  number  of  diorites  are  quartziferous. 


Scheme  of  Deviations  from  Diorite  as  a  Type.      243 

PLATE  V. 


R  2 


244  Descriptive  Petrology. 

The  rock  termed  tonalite  by  Vom  Rath,  which  occurs  in 
the  Tonale  Pass  in  the  Tyrol,  and  which  was  formerly 
regarded  as  a  variety  of  granite,  is  a  micaceous  quartz-diorite. 
The  norite  of  Scheerer  is  a  rock  of  similar  mineral  consti- 
tution, but  it  sometimes  contains  diallage  or  hypersthene 
and  then  passes  over  to  the  gabbros.  The  felspar  is  the 
dominant  mineral  in  norite,  and  sometimes  constitutes 
almost  the  entire  rock.  Norite  occurs  in  the  island  of 
Hitteroe,  off  the  coast  of  Norway,  and  also  in  North  America. 

The  diorites  mostly  occur  as  dykes  and  intrusive  sheets  ; 
the  former  are  usually  fine  grained  at  their  margins  and 
coarsely  crystalline-granular  towards  the  middle  of  the 
dykes.  The  intrusive  sheets  are  sometimes  of  considerable 
extent,  and  follow  more  or  less  closely  the  planes  of  strati- 
fication of  the  rocks,  usually  crystalline  schists,  gneiss,  &c., 
into  which  they  have  been  intruded.  Veins  of  diorite  are 
occasionally  to  be  seen  breaking  through  granite. 

Diorites  are  generally  more  or  less  irregularly  jointed,  but, 
in  some  instances,  a  rude  columnar  structure  is  developed. 
Sometimes  the  diorites  show  a  concentric  spheroidal  struc- 
ture, when  weathered. 

Passages  between  quartz-diorites,  mica-diorites,  &c., 
may  occasionally  be  seen,  and  the  different  varieties  appear 
to  be  mere  local  differentiations  of  the  same  rock. 


CHAPTER  XIII. 

DIABASE   GROUP. 


DIABASE  may  be  regarded  typically  as  a  crystalline-granular 
admixture  of  triclinic  felspar  and  augite,  usually  with  more 
or  less  magnetite  and  titaniferous-iron.  Some  of  these  rocks 
contain  quartz,  and  they  are  consequently  divided  into 


Diabase.  245 

quartz -diabase  and  quartzless-diabase,  or  diabase-proper. 
Chlorite  has  hitherto  been  cited  as  one  of  the  essential  con- 
stituents of  the  rock;  but  Rosenbusch  very  justly  observes  l 
that  if  the  chloritic  matter  in  diabase  be,  as  it  no  doubt  is,  a 
secondary  alteration-product,  it  then  has  only  a  '  patholo- 
gical significance,'  and  cannot,  therefore,  rank  as  an  essential 
constituent.  That  the  viridite  (chlorite,  epichlorite,  or  chlo- 
ritic matter)  in  diabase  is  a  product  of  alteration  is  an 
opinion  now  very  generally  held,  and,  upon  this  ground, 
Allport  has  been  led  to  regard  diabase  as  an  altered  con- 
dition of  dolerite.  According  to  the  researches  of  J.  F.  E. 
Dathe,2  the  felspar  in  diabase  is  not  labradorite,  but  oligo- 
clase.  Rosenbusch  considers  that  diabase  and  gabbro  are 
very  closely  allied,  if  not  identical ;  and  he  bases  this  con- 
clusion upon  the  argument  that  the  essential  difference 
between  these  rocks  is  represented  by  the  statement  that 
diabase  contains  augite,  while  gabbro  contains  diallage ;  and 
he  argues  that  no  essential  difference,  either  chemical,  mor- 
phological, or  optical,  exists  between  these  two  minerals,  and 
that  the  only  appreciable  difference  between  them,  which  he 
is  able  to  recognise,  consists  in  the  fact  that  diallage  shows  a 
structural  condition  of  pinakoidal  separation  due  to  the 
presence  of  twin-lamellation,  or  interpositions,  while,  in 
augite,  no  such  condition  exists.  The  argument  is  most 
masterly;  and  the  reader,  if  interested  in  this  special  question, 
should  consult  the  original  work. 

Diabase  (quartzless-diabase). — The  rock  is  essentially 
crystalline  in  structure,  and  contains  no  trace  of  a  glassy  or 
of  a  devitrified  base.  The  felspar  is  generally  oligoclase, 
and,  according  to  Dathe,  remains  unattacked  when  sections 
of  the  rock  are  treated  with  hot  hydrochloric  acid ;  but,  in 
most  cases,  it  has,  microscopically,  a  hazy,  granulated  appear- 

1  Mik.  Phys.  d.  Massigen  Gesteine.  Rosenbusch,  Stuttgart,  1877, 
P-  323. 

*  Mik.  Untersuch.  uber  Diabase.  J.  F.  E.  Dathe.  Zeitsch.  d.  deutsch. 
Ceol.  Ges.  1874.  Bd.  xxvi.  pp.  1-40. 


246  Descriptive  Petrology. 

ance,  due  to  partial  decomposition.  When  this  condition 
supervenes,  the  crystals  seldom  exhibit  more  than  traces 
of  their  characteristic  twin-lamellation  ;  while,  if  the  change 
be  far  advanced,  all  indications  of  this  structure  become 
obliterated,  and  the  crystals  may  then  at  times  be  mistaken 
for  orthoclase.  In  some  instances  the  triclinic  felspars,  in 
microscopic  sections  of  diabase,  consist  merely  of  two 
lamellae,  thus  resembling  Carlsbad  twins  of  orthoclase ;  and 
occasionally,  according  to  Rosenbusch,  simple,  untwinned 
crystals  of  triclinic  felspar  occur,  which  also  simulate  ortho- 
clase. The  presence  of  true  orthoclase  in  diabase  does  not 
appear  as  yet  to  have  been  established.  The  augite  crystals 
are  generally  seen,  in  microscopic  sections,  to  be  traversed 
by  irregular  fissures  along  which  a  green  decomposition 
product  (viridite),  of  a  scaly,  or  occasionally  a  fibrous  cha- 
racter, is  developed  ;  and  this  also  occurs  along  the  margins 
of  the  crystals.  As  this  decomposition  extends  inwards,  the 
crystals  appear  to  be  irregularly  broken  up  into  irregular 
patches,  in  the  centres  of  which  nuclei  of  unaltered  augite 
still  remain.  When  the  decomposition  is  still  further 
advanced,  no  traces  of  the  original  mineral  are  to  be  de- 
tected, and  the  viridite  constitutes  a  complete  pseudomorph 
after  the  augite.  The  plagioclase  in  these  rocks  seems  also, 
at  times,  to  undergo  a  somewhat  similar  alteration.  Viridite 
also  occurs  interstitially  between  the  different  crystals,  and 
the  entire  rock  frequently  appears  to  be  thoroughly  impreg- 
nated with  this  decomposition  product.  .This  green  sub- 
stance differs  from  chlorite  by  its  more  easy  solubility  in 
hydrochloric  acid.  It  has  been  regarded  by  Gumbel  as 
epichlorite,  a  mineral  of  intermediate  character  between 
chlorite  and  Schillerspar. 

Apatite  occurs  plentifully  in  the  coarser-grained  dia- 
bases :  in  the  more  compact  varieties  it  is  less  common. 
Biotite  is  also  met  with  at  times.  Calcite  is  a  very  common 
secondary  product  in  these  rocks.  Magnetite  occurs  in 
crystals  and  in  fine  grains.  Titaniferous-iron,  iron-pyrites, 


Diabase.  247 

and  copper-pyrites  are  also  minerals  of  common  occurrence 
in  diabase,  and  in  some  few  localities  olivine  forms  one  of 
the  constituents.  The  percentage  of  silica  in  a  diabase 
from  the  Harz  is  44*6,  while  that  in  a  quartz-diabase  from 
Baden  is  53-3; 

Quartz-diabase. — The  constituents  of  this  rock  are  the 
same  as  those  of  quartzl  ess-diabase,  except  that  quartz  and 
biotite  are  always  present.  Quartz-diabase  is  an  essentially 
crystalline-granular  rock,  without  any  interstitial  amorphous 
matter.  It  is,  as  a  rule,  coarse-grained  in  texture.  The 
quartz  occurs  in  small  granules,  seldom  larger  than  a  pin's 
head,  and  fluid  lacunae  are  plentiful  in  them.  Olivine  is 
occasionally  to  be  seen  in  quartz- diabase.  The  diabases, 
both  quartzless  and  quartzose,  show  considerable  variation 
in  their  structural  character.  The  following  are.  some  of  the 
principal  varieties  : — 

Granular-diabase,  in  which  the  individual  constituents 
can  be  recognised  with  the  naked  eye. 

Diabase-aphanite,  a  very  fine-grained  or  compact  variety, 
in  which  the  constituents  are  not  to  be  recognised  without 
the  aid  of  the  lens  or  the  microscope. 

Calc-aphanite  and  Calc-aphanite  schist. — Diabasic  rocks 
which  contain  very  numerous  spherules  of  calcspar,  bordered 
by  chloritic  matter,  and  appearing  to  pass  into  the  surrounding 
matrix. 

The  calc-aphanite  schist  has,  as  its  name  implies,  a 
schistose  structure. 

Diabase-schist  is  also  an  aphanitic  rock  with  a  schistose 
structure. 

Amy gdaloidal- diabase  (Diabasmandelsteiti). — A  vesicular 
diabase,  in  which  the  cavities  have  been  filled  with  calcspar 
by  infiltration.  According  to  von  Lasaulx,  the  character  of 
the  amygdaloids  differs  from  that  of  the  calcareous  spherules 
in  the  calc-aphanites.  To  some  of  these  rocks,  occurring  in 
Nassau  and  in  the  department  of  Haute  Saone,  in  France, 
the  name  spilite  has  been  applied. 


248  Descriptive  Petrology. 

Vdriolite  is  an  aphanitic  diabase  of  compact  texture  and 
greenish- grey  colour,  in  which  there  occur  little  concretions 
of  a  paler  colour,  ranging  up  to  the  size  of  small  nuts.  The 
latter  consist  of  concentric  layers  of  plagioclase,  augite, 
chlorite,  and  epidote.  On  weathered  surfaces  of  the  rock 
these  little  concretions  form  pustular  markings,  whence  the 
name  variolite.  Von  Lasaulx  suggests  that  some  of  the 
rocks  termed  Schalstein  may  be  referred  to  variolite. 

Schalstein  is,  according  to  Giimbel,  a  tuff,  or  sedimentary 
deposit,  the  material  of  which  has  been  derived  from  '  dia- 
base-eruptions.' This  cautious  expression  leaves  it  an  open 
question  whether  the  substance  of  these  tuffs  was  derived 
from  the  disintegration  of  erupted  rock,  or  whether  it 
consists  of  fine  ashy  matter  ejected  from  a  crater.  Certain 
schistose  rocks,  occurring  in  the  neighbourhood  of  Brent  Tor, 
appear  closely  to  resemble  some  of  the  Nassau  Schalstein. 
These  schistose  Devonshire  rocks  were  regarded  by  Sir 
Henry  De  la  Beche  as  volcanic  ash.  In  most  instances 
the  evidence  appears  to  show  that  they  consist  of 
eruptive  matter  of  a  diabasic  character ;  but  whether  this 
finely-divided  matter  was  showered  out  as  ash,  or  whether 
it  resulted  from  the  disintegration  of  an  easily-degraded  rock, 
is  not  very  evident;  while  the  highly  vesicular  and  amygda- 
loidal  character  which  these  beds  sometimes  assume  renders 
it  difficult  to  reconcile  the  co-existence  of  the  vesicular  with 
the  schistose  structure,  unless  the  latter  be  regarded  as  super- 
induced. 1  The  same  doubt  may  be  raised  with  regard  to 
the  amygdaloidal  Schalstein  of  Nassau.  In  Giimbel's  exami- 
nation of  the  diabase  tuffs,  he  mentions  the  appearance  in 
them  of  a  structure  resembling  the  fluxion- structure  seen  in 
many  lavas.  This,  however,  he  attributes  to  a  totally  diffe- 
rent cause — namely,  to  the  re-arrangement  of  detrital  matter 
— and  he  distinguishes  it  by  the  term  '  migration-texture.' 


1  'The  Eruptive  Rocks  of  Brent  Tor,'  Memoirs  of  the  Geological 
Survey  of  England  and  Wales.     Frank  Rutley,  1878,  p.  36. 


Gabbro.  249 

J.  Clifton  Ward  has  pointed  out  the  existence  of  a 
similar  texture  in  some  of  the  rocks  in  Cumberland.1 

The  schistose  diabasic  rocks  contain  a  very  large  pro- 
portion of  green  chloritic  matter,  frequently  in  scales  which 
are  apparently  often  allied  to  sericite. 

The  greenstone-tuffs  and  diabase-tuffs  are  often  closely 
allied  to,  or  identical  with,  the  rocks  just  described. 

GABBRO  GROUP. 

The  gabbros  consist  essentially  of  a  triclinic  felspar, 
generally  labradorite  and  diallage  ;  sometimes,  however,  the 
felspar,  when  altered,  is  represented  by  Saussurite,  and  the 
diallage  by  hypersthene  or  smaragdite,  and  at  times  possibly 
by  enstatite.  Von  Lasaulx  classifies  the  gabbros  in  two 
groups,  the  gabbros  proper  and  the  hypersthenites ;  but  in 
view  of  the  researches  of  Descloizeaux  on  diallage,  and  of 
the  opinions  of  Zirkel,  Rosenbusch,  and  other  petrologists, 
the  hypersthenites,  or  those  rocks  which  consist  of  rhombic 
pyroxene  in  conjunction  with  triciinic  felspar,  are  of  very 
restricted  occurrence.  Since,  however,  the  rhombic  minerals 
hypersthene  and  enstatite  do  occur  in  conjunction  with 
plagioclase  in  a  few  rocks  of  limited  occurrence,  it  seems 
desirable  to  follow  the  arrangement  adopted  by  Rosenbusch, 
and  to  divide  these  rocks  respectively  into  the  plagioclase- 
diallage,  or  true  gabbro  ;  and  the  plagioclase-enstatite,  or 
norite  and  hypersthenite  sub-groups. 

PLAGIOCLASE-DIALLAGE  SUB-GROUP. 

Gabbro. — The  structure  of  the  gabbros  is  crystalline- 
granular  or  granitic,  and  no  interstitial  amorphous  matter 
occurs  in  these  rocks.  Labradorite  and  diallage  are  the 
essential  constituents.  Olivine  is  sometimes  present,  and 
when  this  is  the  case  the  rock  is  distinguished  by  the  term 
olivine-gabbro. 

1  'Geology  of  the  Northern  Parts  of  the  Lake  District,'  Mem. 
Ceo!.  Stirv.  Eng.  and  Wales.     J.  C.  Ward,  1877,  P-  27. 


250 


Descriptive  Petrology. 


The  labradorite  is  usually  pale-grey  or  white,  and  is 
easily  fusible  in  the  blowpipe  flame.  The  crystals  of  labra- 
dorite frequently  show  signs  of  decomposition,  and  then 
contain  green  fibrous  alteration  products  and  opaque-white 
granules,  and  ultimately  pass  into  Saussurite.  The  diallage 
occurs  in  tabular  patches  of  a  grey,  brownish-green,  or 
blackish -green  colour,  with  a  lamellar  structure,  seldom  in 
distinctly-developed  crystals.  The  orthodiagonal  cleavage 
is  well  marked,  and  the  cleavage-faces  usually  present  a 
somewhat  metallic  lustre.  The  diallage  is  frequently  fringed 
with  a  border  of  hornblende.  The  olivine-gabbros  contain 
numerous  dark-greenish  grains  of  olivine,  which  are  usually 
rich  in  microliths.  They  often  show  a  fibrous,  green  alter- 
ation product,  which  probably  represents  their  incipient  con- 
version into  serpentinous  matter. 

Augite,  hornblende,  biotite,  talc,  serpentine,  calcspar, 
pyrites,  pyrrhotine,  garnet,  and  occasionally  quartz  some- 
times occur  as  accessories  in  gabbro.  The  augite  now  and 
then  forms  interlamellations  with  the  diallage,  in  the  same 
way  that  interlamellations  of  monoclinic  and  triclinic  felspars 
sometimes  occur. 

The  three  following  analyses,  cited  by  Von  Lasaulx, 
show  the  general  difference  which  exists  chemically  between 
ordinary  gabbro,  olivine  gabbro,  and  the  so-called  hyper- 
sthenite  of  Penig,  in  Saxony: — 


. 

ii 

iii 

Gabbro 
(Bunsen) 

Olivine-  gabbro 
(Vom  Rath) 

Hypersthenite  (?) 
(Bunsen) 

Si02   =51-35 

50-08 

49-90 

A12O3=  19-82 

I5-36 

16-04 

FeO    =14-95 

672 

— 

Fe203  = 

— 

7-81 

MgO  =  4-14 

9'99 

1  0-08 

CaO   =   3-51 

14-90 

14-48 

K2O    =    2-52 

0-29 

o  *  ^\  ^ 

Na2O  =    3-69 

i  -80 

1-68 

«*>£}  -1'"* 

1-27 

1-46 

Gabbro.  251 

'  A  query  has  here  been  put  against  the  rock  of  Penig, 
since,  although  for  a  long  time  regarded  as  a  typical  hyper- 
sthenite,  it  has  since  been  suggested  by  Zirkel  that  the  mine- 
ral in  this  rock,  hitherto  considered  to  be  hypersthene,  must 
now  be  reckoned  as  diallage,  its  almost  total  absence  of 
dichroism  precluding  the  supposition  that  it  is  hypersthene.1 
He  also  states  that  the  supposed  hypersthene  in  the  so-called 
hypersthenites  of  Veltlin,  Neurode,  and  the  Isle  of  Skye,  is 
simply  diallage. 

PLAGIOCLASE-ENSTATITE  SUB-GROUP. 

The  rocks  of  this  group  appear  to  differ  very  little  in 
mineralogical  constitution  from  ordinary  gabbros,  except 
that  their  pyroxenic  constituent  is  rhombic  and  not  mono- 
clinic.  Considerable  difficulty  frequently  attends  the  dis- 
crimination between  hypersthene,  enstatite,  and  bronzite, 
and  it  is  therefore  at  times  very  unsafe  to  express  any  strong 
and  decided  opinion  as  to  the  precise  nature  of  the  rhombic 
mineral  which  represents  the  pyroxenic  constituent  of  these 
rocks,  which  appear  to  be  generally  massed  by  Rosenbusch 
under  the  name  norite.  The  rock  of  St.  Paul's  Island,  on 
the  coast  of  Labrador,  in  which  the  most  typical  hypersthene 
occurs,  is  placed  by  this  author  among  the  diallage-  and 
olivine-bearing  hypersthene  norites.  The  norites  of  Hitteroe 
consist  of  plagioclase  and  hypersthene,  in  which  the  inter- 
posed plates,  &c.,  so  characteristic  of  the  typical  hypersthene, 
are  very  generally  absent.  These  rocks  also  contain  a  little 
orthoclase  and  diallage.  Olivine  and  mica  occur  in  some  of 
the  norites,  and  bronzite  has  been  recorded  in  one  or  two 
localities  as  an  essential  constituent. 

Serpentine  and  Schillerspar  (Bastite)  are  sometimes 
present  in  these  rocks  when  they  are  more  or  less  weathered. 
The  norites  never  contain  any  glassy  matter. 

Gabbro  occurs  in  the  form  of  intrusive  masses,  often  of 

1  Mikroskop.  Beschaff.  d.  Min.  u.  Gest.  Zirkel.  Leipzig,  1873, 
p.  181. 


252  Descriptive  Petrology. 

considerable  magnitude,  and  in  dykes,  veins,  and  intrusive 
sheets,  which  are  sometimes  forced  along  the  planes  of 
bedding  in  the  adjacent  stratified  rocks. 

BASALT  GROUP. 

iDolerite,  anamesite,  and  basalt,  or  basaltite,  are  names 
applied  to  the  rocks  of  this  group,  which  imply  different 
conditions  of  texture  and  crystalline  development,  rather 
than  any  marked  difference  in  mineralogical  constitution  or 
chemical  composition.  Still  some  difference  between  them 
frequently  exists  in  the  relative  percentages  of  silica  which 
they  contain,  and  also  in  their  specific  gravities?) 

(The  rocks  of  the  basalt  group  all  contain  augite,  magne- 
tite, and  titaniferous  iron  (of  the  last  two  minerals  sometimes 
one,  sometimes  both  are  present),  but  they  have  in  addition 
other  mineral  constituents  which  generally  form  a  very 
considerable  proportion  of  the  rock)  and  indeed  in  some 
instances  play  quite  a  dominant  part.  Of  these  the  felspars 
may  claim  the  most  prominent  place.  They  are  triclimc. 
Monoclinic  felspar,  although  met  with  at  times,  is  of  com- 
paratively exceptional  occurrence.  ^Olivine,  nepheline,  and 
leucite  are  minerals  which  exist  very  plentifully  in  some  of 
the  basalts  \  in  the  constitution  of  others  they  occupy  quite 
a  subordinate  place  ;  while  in  some,  again,  they  are  totally 
absent.  The  occurrence  of  leucite  seems  to  be  restricted  to 
certain  localities,  and  this  mineral  has  not  as  yet  been  de- 
tected in  any  British  rocks. 

Hauyne  and  nosean  (which  latter  may  be  included  under 
the  former  name)  are  sometimes  sparsely  disseminated ;  at 
other  times  they  occur  in  such  considerable  quantity  as  to 
give  a  distinctive  character  to  the  rock. 

(Micas  occur  rather  plentifully  in  some  of  the  basalts, 
occasionally  to  such  an  extent  as  to  impart  a  special 
character  to  them.) 

The  basalts  have  been  conveniently  classified  by  Mohl 


Basalt.  253 

according  to  their  mineral  constitution,  in  the  following 
manner  : — 

i.  Magma-basalts,  with  a  colourless  or  brown  glass  matrix, 
ii.  Plagioclase-basalts,  containing  notably  plagioclase  and 

occasionally  nepheline  in  addition  to  the   essential 

augite,  magnetite,  &c.     Leucite  seldom, 
iii.  Nepheline-basalts,  containing    notably    nepheline,    and 

sometimes  leucite,  in  addition  to  augite,  magnetite,  &c. 

Plagioclase  rare  or  absent. 
iv.  Leucite-basalls. 
v.  Hauyne-  and  nosean-basalts. 
vi.  Mica-basalts. 

The  old  term  divine-basalt  is  not  included  in  this 
classification,  apparently  for  the  reason  that  olivine  may,  and 
very  commonly  does,  occur  to  a  greater  or  less  extent  in  all 
of  the  basalts. 

The  rocks  termed  magma -basalts  have  already  been 
alluded  to  under  the  name  augite-tachylyte.1 

(The  basalts  vary  considerably  in  structure  :  the  coarsely 
crystalline  varieties,  and  those  in  which  the  different  mineral 
constituents  are  sufficiently  well  developed  to  be  distinguished 
by  the  naked  eye,  are  termed  dolerite^  those  in  which  the 
constituents  are  too  small  to  be  recognised  without  a  magni- 
fying power,  but  in  which  a  crystalline  texture  is  yet  clearly 
discernible,  are  styled  anamesites  /while  the  still  more  com- 
pact varieties,  which,  to  unassisted  vision,  present  a  more 
or  less  homogeneous  appearance,  are  called  basalts  (basalts 
proper)  or  basaltites. 

Plagioclase  basalts. — The  constituents  of  these  rocks  are 

1  Boftcky  classifies  the  basalts  as 

Melaphyr-basalt, 

Felspar-basalt, 

Phonolite-  and  andesite-basalt, 

Trachy-basalt, 

Tachylyte-basalt. 

Rosenbusch  considers  that  most  of  the  rocks  included  in  Boficky's 
last  three  groups  are  more  or  less  closely  allied  to  the  tephrites,  or  those 
rocks  which  are  characterised  by  the  presence  of  nepheline  or  leucite 


254  Descriptive  Petrology. 

plagioclastic  felspars,  augite,  and  magnetite.  Titaniferous 
iron  is  frequently  present.  Apatite,  olivine,  nepheline, 
and  hauyne  may  also  be  accessory:  Carbonate  of  iron, 
calcspar,  zeolites,  chalcedony,  &c.,  occur  as  secondary 
products,  and  very  commonly  fill  the  interior  of  vesicles. 
The  spaces  between  the  individual  crystals  are  often  filled 
with  a  glass-magma,  usually  of  a  brownish  tint,  and  fre- 
quently containing  great  numbers  of  opaque  trichitesi  As 
a  rule,  the  glassy  matter  represents  only  a  very  small  pro- 
portion of  the  entire  rock.  The  plagioclase  in  these  rocks 
is  sometimes  oligoclase,  sometimes  labradorite,  anorthite,  or 
andesine.  It  is,  however,  in  most  cases  oligoclase.  Ortho- 
clase  also  occurs  at  times  in  these  rocks,  but  its  presence  is 
quite  exceptional.  Olivine  frequently  forms  an  important 
constituent  of  the  plagioclase  basalts. 

In  microscopic  sections  of  basalts  which  have  undergone 
partial  decomposition,  the  olivine  and  augite  crystals  are 
often  merely  represented  by  pseudomorphs  of  green  matter, 
which  is  serpentine  or  some  other  hydrous  silicate.  The 
augite  in  basalts  is  generally  rich  in  glass  inclosures.  Steam 
pores  and  fluid  lacunae  are  also  of  common  occurrence  in 
*them.  The  olivine  sometimes  appears  in  tolerably  well- 
defined  crystals  ;  but  it  is  more  usually  in  roundish  grains,  or 
in  granular  aggregates.  The  latter  are  sometimes  of  con- 
siderable size,  and  occasionally  show,  in  external  configu- 
ration, that  they  are  large,  rudely-developed  crystals.  The 
plagioclase  basalts  are  of  more  frequent  occurrence  than  any 
of  the  other  rocks  belonging  to  the  basalt  group. 

in  conjunction  with  plagioclase.  Rosenbusch  defines  basalt  as  a  rock 
consisting  essentially  of  olivine,  augite,  and  plagioclase,  and  regards 
these  rocks  as  the  tertiary  and  recent  equivalents  of  olivine-diabase  and 
melaphyre.  Sandberger  has  proposed  a  division  of  these  rocks  into 
those  which  contain  titanic-iron  and  those  which  contain  magnetite. 
The  former  he  designates  dolerites,  the  latter  basalts.  This  classifica- 
tion, however,  as  suggested  by  Rosenbusch,  is  by  no  means  satisfactory, 
owing  to  the  frequent  difficulty  in  distinguishing  between  these  two 
minerals,  and  also  from  the  fact  that  magnetite  is  very  commonly 
titaniferous. 


Basalt.  255 

Von  Lasaulx  gives  the  two  following  analyses  as  repre- 
senting the  average  composition  of  a  coarsely  crystalline  and 
of  a  compact  variety,  the  former  being  a  doleritic,  the  latter 
a  basaltic,  type  :  — 

Plagioclase  dolerite  :  SiO3  =  50-59,  A12O3  =  14-10,  Fe2O3  =  16-02, 
CaO  =9-20,  MgO  =  5-09,  K2O  =  1-05,  Na2O  =  2-19, 


Plagioclase  basaltite  :  Si02  =  43'O,  A12O3  =  14-0, 

Fe2O3  and  FeO  =  15-30,  CaO  =  12-10,  MgO  =  9-10, 
K2O  =  i  -30,  Na2O  -  3-87,  H2O  =  I  -30. 

The  lavas  of  Etna  appear  for  the  most  part  to  be 
plagioclase  basalts,  rich  in  olivine.  The  plagioclase  crystals 
in  these  lavas  contain  great  numbers  of  irregularly-shaped 
glass  inclosures. 

Nepheline  basalt,  or  nephelinite.-(This  is  a  crystalline 
granular  admixture  of  nepheline,  augite,  and  magnetite. 
More  or  less  olivine  is  always  present.)  Apatite,  sphene, 
hauyne,  melilite,  and  garnet  are  among  the  more 
common  accessory  minerals.  The  nephelinite  of  Katzen- 
buckel  in  the  Odenwald,  described  by  Rosenbusch,1  may 
be  taken  as  one  of  the  most  typical  examples.  (  Only  mere"* 
traces  of  interstitial  glass  are  ever  to  be  seen  in  these  rocks  r 
some  however  contain  interstitial  nepheline,  which  may  be 
easily  distinguished  from  glassy  matter  by  its  polarisation, 
and  by  the  crystalline  aggregate  character  of  the  patches, 
although  no  definitely  developed  crystals  may  be  visible^ 

The    following    is    an   analysis    of   the  nephelinite   of 
Katzenbuckel  by  Rosenbusch  :  — 

SiO2  =  42'3,  A12O3  and  Fe2O3  =  28-o,  CaO  and  MgO  =  13-65, 


\The  rock  also  contains  0-65  per  cent,  of  phosphoric 
acid,  and  traces  of  the  oxides  of  nickel,  cobalt,  and 
manganese) 

1  Der  NcpheKnit  vom  Katzenbuckel.     Freiburg,  1869. 


256  Descriptive  Petrology. 

t 

iLeucite,  sodalite,  and  sanidine  are  occasionally  met  with 
as  accessories  in  nepheline  basalts.  These  rocks  sometimes 
assume  a  very  vesicular  character,\as  in  the  millstone-lavas 
of  the  Eifel,  and  of  Niedermentlig  on  the  Rhine.  The 
latter  rock  is,  however,  often  so  rich  in  hauyne  that  it  may 
rather  be  classed  in  the  sub-group  of  hauyne-basalts. 

(.These  vesicular  rocks  assume  an  amygdaloidal  charac- 
ter when  the  vesicles  are  filled  with  various  minerals!) 

Leucite-basalt  (Leucitophyr,  Leucilite).-(;The  rocks  of 
this  sub-group  are  seldom  coarse-grained,  and  are  mostly  of 
a  greyish  colour,  the  leucite  crystals  often  giving  them  a 
light  speckled  appearance.  They  are  essentially  aggregates 
of  leucite,  augite,  and  magnetite^  Olivine  and  nepheline 
are  very  generally  present,  sometimes  in  considerable  quan- 
tity. Nosean  is  sometimes  plentiful,  and  biotite  and  sphene 
also  occur  as  accessories.  ^Under  the  microscope,  scarcely 
any  trace  of  vitreous  matter  is  ever  to  be  detected  in  the 
leucite-basalts)  unless  the  leucite-sanidine-lavas  of  Vesuvius 
may  be  included  under  this  name.  In  most  of  these  rocks 
felspars  are  totally  absent,  although,  in  some  of  the  leuci- 
tophyrs  of  Vesuvius  and  the  Eifel,  sanidine  crystals  are  met 
with  of  tolerably  large  dimensions.  In  all  the  rocks  of  this 
sub-group,  leucite  is,  as  a  rule,  the  dominant  constituent.  In 
some  of  the  leucitophyrs,  as  for  example  in  the  rock  termed 
sperone,  which  occurs  in  the  neighbourhood  of  Rome,  the 
leucite  constitutes  almost  the  entire  mass,  and  the  crystals, 
which  are  mostly  of  minute  size,  are  very  closely  packed 
together.  These  crystals,  when  very  small,  no  longer  exhibit 
their  characteristic  crystalline  form,  but  appear  under  the 
microscope  as  round  spots  having  rather  ill-defined  boun- 
daries. >(  The  leucite  crystals  are  generally  rich  in  interposi- 
tions,) such  as  those  previously  described  at  page  no. 

The  leucite-sanidine  lavas  of  Vesuvius  have,  as  a  rule, 
such  a  very  complex  mineralogical  constitution,  that  they 
cannot  be  regarded  as  the  equivalents  of  basalts.  They 
number  among  their  constituents  leucite,  sanidine,  plagio- 


Basalt.  257 

clastic  felspar  [mainly  anorthite],  nepheline,  sodalite,  hauyne, 
augite,  hornblende,  olivine,  biotite,  apatite,  &c.  The  majority 
of  the  Vesuvian  lavas  consist  of  seven  or  eight  of  these 
minerals.  An  account  of  them  will  be  found  in  the  Trans- 
actions of  the  Royal  Irish  Academy,  vol.  xxvi.  * 

Hanyne-basalt  (Hauynophyr). — Leucite,  nepheline, 
hauyne,  augite,  and  magnetite  are  the  principal  constituents, 
with  usually  some  olivine  and  apatite.  Vitreous  matter 
occurs  sparingly  in  these  rocks  and  generally  contains  nume- 
rous trichites.  Felspars,  both  monoclinic  and  triclinic,  are 
absent.  The  hauyne  crystals,  which  for  the  most  part  are 
blue,  but  also  greyish  or  colourless  at  times,  although  fre- 
quently small,  are  seldom  of  very  minute  dimensions.  Some- 
times the  rock  assumes  a  porphyritic  character,  through  the 
increased  development  of  hauyne  and  augite.  The  most 
typical  examples  of  hauyne-basalt  occur  at  the  Laacher  See 
in  the  Eifel,  and  at  Melfi  near  Naples.  In  the  rock  at  the 
latter  locality  the  hauyne  crystals  sometimes  appear  red, 
owing  to  the  interposition  of'  lamellae  of  hematite.  This  red 
colour  does  not,  however,  always  extend  to  the  surface,  so 
that  the  fractured  crystals  sometimes  have  a  red  nucleus 
surrounded  by  a  blue  border.  Hauyne-basalts  are  rocks  of 
very  limited  occurrence. 

Mica-basalts. — These  can  scarcely  be  regarded  as  a 
distinct  sub-group,  since  the  mica  which  they  contain  does 
not  exclude  the  occurrence,  and  cannot  be  considered  as 
the  representative,  of  any  of  the  essential  constituents  of  the 
sub-groups  already  described,  unless,  in  any  cases,  its  mode 
of  occurrence  could  be  reconciled  with  the  observations  of 
Kjerulf  on  the  mica-pseudomorphs  after  augite,  which  he 
procured  from  the  Eifel ;  or  those  of  J.  D.  Dana,  on  the 
alteration  of  olivine  into  mica.2  The  mica-basalts  are  rocks 
pertaining  to  the  plagioclase,  the  nepheline,  the  leucite,  or 

1  '  Report    on    the   Chemical,    Mineralogical,    and   Microscopical 
Characters  of  the  Lavas  of  Vesuvius  from  1631  to  1868,'  by  Professors 
Haughton  and  Hull.     Dublin,  1876. 

2  System  of  Mineralogy,  J.  D.  Dana,  1871,  p.  258. 

S 


258  Descriptive  Petrology. 

the  hauyne  basalts,  and  since  any  or  all  of  these  rocks  may 
at  times  contain  mica  as  an  accessory,  the  only  distinction 
which  exists  between  them  and  the  mica  basalts  appears  to 
be  summed  up  in  the  statement  that  mica  basalts  are  rich  in 
mica,  while  the  other  basalts  contain  that  mineral  in  very 
limited  quantity,  or  as  an  accessory.  The  mica  crystals  in 
these  rocks  vary  considerably  in  size ;  sometimes  they  are 
quite  large,  at  others,  they  occur  as  fine  microscopic  scales, 
distributed  very  closely  and  uniformly  through  the  rock. 
These  micas  are  mostly  dark  brown,  reddish  brown,  or 
black,  and  may,  in  many  cases,  be  referred  to  biotite. 

The  basalts  occur  in  lava  streams,  plugs,  intrusive  sheets 
('  Whin  Sill '  of  the  north  of  England),  and  dykes.  They 
are  often  traversed  by  structural  planes  which  are,  in  some 
cases,  so  disposed  that  the  rock  assumes  a  columnar  charac- 
ter, as  at  the  Giant's  Causeway,  Fingal's  Cave,  and  at  many 
foreign  localities.  The  columns  are  occasionally  curved. 
They  sometimes  stand  in  vertical,  at  others  in  horizontal  or 
inclined  positions,  which,  in  all  cases,  are  directed  at  right 
angles  to  the  surfaces  upon  which  the  rock  cooled.  This 
columnar  structure  is  caused  by  the  contraction  of  the 
basalt  on  cooling,  but  it  is  not  exclusively  in  basalts  that  it 
occurs  ;  it  is  occasionally  to  be  met  with  in  trachytes,  phono- 
lites,  pitchstones,  felstones,  also  in  argillaceous  rocks  at 
their  contact  with  eruptive  masses.1  Sometimes  a  platy  or 
tabular  structure  is  developed  in  basalt,  especially  near  the 
margins  of  intrusive  plugs  or  dykes.  Spheroidal  structure 
also  occurs  in  these  rocks,  and  the  spheroids  or  balls  may  be 
seen  often  closely  packed  between  the  divisional  planes 
which  constitute  the  boundaries  of  the  columns.  The 

1  Some  interesting  experiments  were  made  by  Mr.  W.  Chandler 
Roberts  in  connection  with  the  artificial  production  of  columnar  struc- 
ture, and  he  has  kindly  supplied  the  following  note.  '  A  mixture  of 
clay  and  sand,  in  the  form  of  Windsor-brick,  was  heated  to  about 
1020°  c.  and  slowly  cooled.  The  mass  was  found  to  have  contracted 
by  about  6  per  cent,  (cubical),  and  columnars  tructure  was  well  deve- 
loped in  it.' 


Basalt. 


259 


columns  are  sometimes  divided  by  cup-like  joints,  so  that 
one  portion  of  the  column  is  convex  and  fits  into  a  concave 
surface  on  the  adjacent  part  of  the  column.     The  number 
FIG.  87.  of  sides,  which  basalt 

columns  present,  varies. 
Occasionally  they  have 
only  three  sides,  at 
other  times  five,  six,  or 
eight,  as  shown  in  the 
accompanying  figure 
87.*  The  subjoined 
papers  on  these  struc- 
tures may  be  consulted 
with  advantage.2  Basalt  occurs  in  the  form  of  wide-spread 
lava  flows,  and  coulees  or  streams,  in  dykes,  in  irregular 
bosses,  and  in  plugs  or  pipes,  which  represent  the  filled-up 
flues  or  feeders,  from  which  lava  streams  were  once  poured 
out. 

1  From  illustrations  in  the  late  G<  V.  Du  Noyer's  'Notes  on  the 
Giant's   Causeway,'   The  Geologist,  vol.   iii.   1860.      'It   appears   now 
to  be  pretty  certainly  established  that  the  peculiar  structure  of  columnar 
basalt  is  due  to  contraction  and  splitting,  consequent   upon  cooling. 
The  idea  entertained  by  some  of  the  older  geologists,  that  the  hexagonal 
form,  so  frequently  found,  was  caused  by  the  squeezing  together  of  masses 
originally  spherical,  is  geometrically  incorrect.     This   process  would 
give  rise  to  rhombic  dodecahedra,  more  or  less  regular,  and  could  under 
no  circumstances  lead  to  six-sided  columns.    The  cup-shaped  joints,  so 
frequently  found,  have  also  been  shown  to  be  a  natural  consequence  of 
the  contraction  on  cooling,  to  which  the  columnar  structure  is  ascribed. 
In  this  view,  the  analogy  of  columnar  basalt  is  rather  to  the  splitting, 
often  seen  in  the  mud  bottom  of  a  dried-up  pool,  than  to  ordinary 
crystallisation.     The  direction  of  the  columnar  axis  with  reference  to 
the  apparent  planes  of  cooling — the  confusion  of  structure  towards  the 
middle  of  the  dykes  or  beds — the  cup  joints— the  irregularity  of  the 
prisms,  whose  cross  sections  are  seldom  regular  hexagons — the  way  in 
which  a  hexagon  passes  into  a  pentagon  through  a  heptagon,  and  not 
directly — all  point  to  the  contractile  origin  of  the  structure,  at  the  same 
time  that  the  result  suggests  a  curious  mimicry  of  imperfect  crystallisa- 
tion.'—C.  W.  M. 

2  Gregory  Watt,   'Observations   on  Basalt,'  Phil. .  Trans.     1804, 
pp.  279-313. 

Scrope  '  On  Volcanoes.' 

James  Thomson,  'On  the  Jointed  Prismatic  Structure  of  the  Giant's 
s  2 


260  Descriptive  Petrology. 

Melaphyre. — The  precise  grounds  upon  which  the  rocks 
termed  melaphyre  have  been  raised  to  the  dignity  of  a 
distinct  petrological  group  are  by  no  means  apparent. 
Rosenbusch  seems  to  regard  them  as  closely  related  to,  if 
not  identical  with,  olivine-diabase.  It  is  evidently  a  some- 
what doubtful  question  whether  they  should  be  classed  with 
diabase  or  basalt.  Melaphyre  may  be  denned  as  a  fine 
grained  or  compact,  black,  greenish-black,  or  brownish-black 
aggregate  of  plagioclase,  augite,  olivine,  magnetite,  or  titani- 
ferous  iron,  and  delessite  or  chlorophoeite.  These  two  last 
constituents  are  considered  to  distinguish  melaphyre  from 
basalt,  [but  melaphyres  possess  a  vitreous,  or  a  devitrified, 
magma  which  allies  them  more  to  basalt  than  to  diabase]. 

Now,  delessite  is  a  ferruginous  chlorite,  and  chlorophceite 
is  a  hydrous  silicate  of  protoxide  of  iron,  also  allied  to 
chlorite,  or  embraced  by  that  very  comprehensive  term. 
Both  of  these  minerals  are  decomposition  products,  and  it 
therefore  appears  that  their  presence  should  serve  to  render 
the  true  nature  of  the  rock  a  matter  of  doubt,  rather  than  to 
constitute  one  of  its  distinctive  characters. 

Allport's  suggestion  that  melaphyre  should  be  included 
in  the  term  dolerite,  of  which  he  regards  it  simply  as  a 
partially- altered  condition,  seems  at  least  plausible.1  The 
definition  given  by  Boficky  in  the  introduction  to  his 
1  Petrographische  Studien  an  den  Melaphyr-Gesteinen 
Bohmens,' 2  appears  to  a  great  extent  to  confirm  the  foregoing 
statement.  The  melaphyres  are  of  palaeozoic  age,  and  this 

Causeway,'  read  at  the  British  Association  Meeting  at  Belfast  in  1874, 
but  only  the  title  given  in  the  report. 

R.  Mallett,  '  On  the  Origin  and  Mechanism  of  Production  ...  of 
Basalt,'  R.  S.  Proceedings,  1874-5,  vo1-  xxiii-  PP-  180-84. 

T.  G.  Bonney,  '  On  Columnar,  Fissile,  and  Spheroidal  Structure,' 
Q.  y.  G.  S.,  vol.  xxxii.  p.  140,  1876. 

1  'On  the  Microscopic  Structure  and  Composition  of  British  Car- 
boniferous Dolerites,'  by  S.  Allport,  Quart.  Journ.  Geol.  Soc.  vol.  xxx. 

P-  530. 

2  Archiv  d.  Nat.  Wiss.  Landesdurchforsch.  v.  Bohmen,  Geol.  Abth. 
bd.  iii. 


Scheme  of  Deviations  from  Basalt  as  a  Type.    261 

PLATE  VI. 


NEPHELINE         FELSPAR 

(Triclinic) 


LEUCfTE 


»4      Neplieline  Basalb* 
Q 


MAGNETITE 
&    TITANIFEROUS    IRON 


262  Descriptive  Petrology. 


fact  seems  to  have  been  one  of  the  very  insufficient  reasons 
for  separating  them  from  similar  rocks  of  later  date. 

The  vitreous  conditions  of  basalt  have  been  already  de- 
scribed under  the  head  of  tachylyte. 

Basalts  often  assume  a  vesicular  character,  which  is 
generally  most  prevalent  at  and  near  the  upper  and  the 
lower  parts  of  the  lava  streams.  The  vesicles,  when  subse- 
quently filled  with  calcite,  zeolites,  and  other  minerals  of 
secondary  origin,  render  the  rock  amygdaloidal.  Some  of 
the  basalts  (toadstones)  of  Derbyshire  show  this  character 
very  well. 

Basalts  occur  of  various  ages,  ranging  upwards  into  Ter- 
tiary, and  Post-tertiary  times. 

Basalts  of  Dimetian  age  have  been  identified  by  Pro- 
fessors Judd  and  Bonney,  and  also  by  Mr.  Tawney.1 

ROCKS  OF  EXCEPTIONAL  MINERAL  CONSTITUTION. 
( Characterised  by  the  absence  of  felspars) 

Garnet-rock. — A  crystalline-granular  aggregate  of  garnet 
and  hornblende,  usually  with  more  or  less  magnetite.  The 
garnets,  as  a  rule,  constitute  a  far  larger  proportion  of  the 
rock  than  the  other  minerals.  They  may  very  commonly 
be  referred  to  the  iron-lime  varieties,  and  are  mostly  of  a 
brownish  or  yellowish  colour.  Other  minerals  are  also  fre- 
quently present,  such  as  epidote  and  calcite.  The  garnet- 
rocks  are  of  very  limited  occurrence,  and  are  chiefly  met 
with  in  Saxony,  Bohemia,  the  Urals,  and  Canada,  forming 
irregular  veins  in  mica-schist. 

Kinzigite. — A  crystalline  aggregate  of  spessartine  (man- 
ganese garnet),  magnesian-mica  and  oligoclase,  often  con- 
taining some  iolite  and  fibrolite,  the  latter  a  monoclinic  mine- 
ral, having  a  chemical  composition  identical  with  that  of 
andalusite.  It  occurs  at  Wittichen,  at  the  Kinzig,  Schwarzwald. 

1  '  On  the  Older  Rocks  of  St.  David's,'  by  E.  B.  Tawney,  Ptoc. 
Bristol  Nat.  Soc.  vol.  ii.  p.  113. 


Eklog  ite.     L  herzolite.  263 

Eulysite. — An  aggregate  of  reddish-brown  garnet,  green 
augite,  and  a  mineral  which,  in  chemical  composition,  is  allied 
to  the  iron-olivine,  fayalite.  The  last-named  mineral  is  the 
dominant  constituent  of  the  rock.  Eulysite  occurs  in  a  very 
thick  bed  in  the  gneiss  of  Tunaberg  in  Sweden. 

Eklogite  (Disthene-rock). — A  granular  aggregate  of  red 
or  reddish- brown  garnet,  smaragdite  (a  green  variety  of 
diallage  which,  according  to  Descloizeaux,  has  the  cleavage 
and  optical  properties  of  amphibole),  hornblende,  or  ompha- 
cite  (a  grass-green  variety  of  pyroxene  with  two  sets  of 
cleavage,  one  more  perfect  than  the  other,  intersecting  at 
an  angle  of  115°).  Kyanite  (disthene),  silvery  white  mica, 
quartz,  olivine,  zircon,  apatite,  sphene,  oligoclase,  and  py- 
rites, also  occur  at  times  as  accessories.  The  eklogite  from 
Eppenreuth  contains  about  70  per  cent,  of  omphacite  and 
25  of  garnet.  Other  varieties,  such  as  those  from  the  Fichtel- 
gebirge  and  Baden,  are,  on  the  other  hand,  particularly  rich 
in  hornblende.  Others  again  contain  a  large  proportion  of 
disthene,  mica,  and  quartz,  and  on  this  account  may  prefer- 
ably be  termed  disthene-rock. 

The  garnets  in  eklogite  are  often  surrounded  by  an 
envelope  of  bright-green  hornblende,  while  brown  and  feebly  - 
dichroic  hornblende  also  occurs  in  the  same  rock. 

The  freshly  broken  surfaces  of  the  rock  present  a  very 
beautiful  appearance  from  the  juxtaposition  of  red  garnets 
with  bright  green  omphacite. 

Lherzolite. — A  granular  or  crystalline-granular  aggregate 
of  olivine,  enstatite,  diopside,  and  picotite  (a  black  spinel, 
containing  over  7  per  cent,  of  sesquioxide  of  chromium). 
The  olivine  is  the  dominant  constituent.  The  rock  varies 
considerably  in  texture;  in  some  instances  it  is  coarsely 
granular  and  feebly  coherent,  crumbling  when  handled ;  in 
others  it  is  of  a  medium  crystalline-granular  character,  and 
quite  tough.  The  enstatite  is  of  a  greenish-brown  or  yellow 
colour.  In  thin  sections  it  appears  almost  colourless  by 
ordinary  transmitted  light.  It  has  a  more  or  less  fibrous 


264  Descriptive  Petrology. 

aspect.  The  cleavages  parallel  to  oo  P  intersect  at  an  angle 
of  about  87°;  less  distinct  cleavages  parallel  to  the  pinakoids 
are  also  visible,  and  are  generally  rendered  more  apparent 
by  rotating  the  section  between  crossed  Nicols.  The 
diopside  has  a  rough  or  stepped  appearance  on  the  abraded 
surfaces  of  sections,  and  shows  the  characteristic  cleavage 
of  augite.  It  occurs  in  roundish,  green  grains.  The 
picotite  appears,  under  the  microscope,  in  very  irregular 
brown,  or  (according  to  Bonney)1  deep  olive-green,  patches 
or  grains,  which,  in  aspect,  somewhat  resemble  dots  and 
streaks  of  some  gummy  substance.  They  appear  dark 
between  crossed  Nicols. 

The  olivine  is  very  frequently  altered  into  serpentine,  the 
process  of  decomposition  taking  place  in  the  first  instance 
along  the  cracks  in  the  olivine  grains  and  crystals ;  and,  as 
it  advances,  they  become  traversed  by  a  mesh-work  of  little 
strings  of  serpentinous  matter,  until,  in  the  final  stage,  no 
olivine  remains,  the  rock  often  being  impregnated  with  this 
decomposition  product  to  such  an  extent  that  it  is  virtually 
a  serpentine  rock,  as  pointed  out  by  Von  Lasaulx,  and  more 
fully  described  by  Bonney,  who  states  his  belief  that  Iherzo- 
lite  is  an  intrusive  rock. 

Rosenbusch,  in  describing  the  extreme  phases  of  altera- 
tion into  serpentine,  remarks  that  the  pseudomorphs  after 
the  enstatite  and  olivine  may  be  microscopically  distin- 
guished from  one  another  by  the  rectangular,  grating-like 
disposition  of  the  fibrous  structure  in  the  serpentine,  re- 
sulting from  the  alteration  of  enstatite  or  diallage,  and  the 
irregular  character  of  the  fibrous  mesh-work  which  is  set  up 
in  the  decomposed  olivine. 

Lherzolite  occurs  in  veins  of  limestone  at  the  Etang  de 
Lherz,  in  the  Eastern  Pyrenees,  whence  it  takes  its  name. 
It  is  also  met  with  in  the  Tyrol,  the  department  of  Haute  - 
Loire,  Nassau,  Norway,  &c.  Pyrope  occurs  as  an  accessory 

1  'The  Lherzolite  of  the  Adage,'  GeoL  Mag.  decade  ii.  vol.  iv. 
p.  64. 


D  unite.     Picrite.  265 

in  a  serpentinous  condition  of  this  rock,  in  certain  localities. 
The  olivine  bombs  met  with  in  some  basalts  are,  according 
to  Von  Lasaulx,  closely  akin  to  Iherzolite.  The  chemical 
composition  of  a  Norwegian  Iherzolite  is  cited  by  that 
author  as 

SiO2  =  37-42,  A12O3  =  o-io,  MgO  =48-22,  FeO  =  8-88,  MnO  - 
0-17,  NiO=o-23,  H2O=o7i. 

Dunite  (so  named  from  Dun  Mountain  in  New  Zealand, 
which  consists  in  great  part  of  this  rock  and  serpentine), 
is  a  crystalline-granular  aggregate  of  olivine  and  chromic- 
iron  ;  the  former  occurring  in  yellowish-green  grains  and  the 
latter  in  black  octahedra.  Dunite  passes  by  alteration  into 
serpentine.  The  frequent  association  of  chromic-iron  with 
serpentine  renders  it  probable  that  many  serpentines  may 
have  resulted  from  the  alteration  of  some  rock  analogous 
in  mineral  constitution  to  dunite.  This  rock  also  occurs 
in  the  south  of  Spain  and  in  several  other  European  loca- 
lities. 

Diallage  and  enstatite  are  present  in  small  quantities  in 
some  varieties  of  dunite,  which,  under  these  circumstances, 
approximates  to  Iherzolite. 

Picrite  is  a  blackish-green  crystalline  rock  with  a 
compact,  black  matrix,  containing  porphyritic  crystals  and 
grains  of  olivine.  The  matrix  may  consist  of  hornblende, 
diallage,  or  biotite,  associated  with  magnetite  and  calcspar. 
The  olivine  constitutes  nearly  half  the  bulk  of  the  rock.  A 
small  amount  of  vitreous  matter  containing  microliths  is 
sometimes  present. 

•  Schorl-rock,  although  previously  mentioned  in  the  de- 
scription of  the  granitic  rocks,  may  also  be  placed  in  this 
miscellaneous  group.  The  constituents  are  schorl  and 
quartz.  Topaz,  mica,  and  tinstone  sometimes  occur  as 
accessories.  It  is  intimately  connected  with  granite,  and, 
by  the  accession  of  orthoclase  and  mica,  passes  into  the 
schorlaceous  varieties  of  that  rock. 


266  Descriptive  Petrology. 

VOLCANIC  EJECTAMENTA. 

These  comprise  dust,  ashes,  sand,  lapilli,  and  volcanic 
bombs.  They  all  consist  of  mineral  matter  which  has 
undergone  a  variable  amount  of  trituration  and  which  has 
been  ejected,  either  in  a  solid  condition,  or  in  a  state  of 
fusion.  The  expulsion  of  this  matter  from  craters  is  due  to 
the  explosions  of  steam  and  gases  which  occur  within  the 
volcanic  vents.  The  lava,  which  is  in  a  fused  and  viscid  or 
pasty  condition,  naturally  becomes  injected  more  or  less 
completely  with  steam  and  gases,  the  bubbles  of  which,  when 
imprisoned  in  the  molten  masses  and  unable  to  escape, 
produce  a  vesicular  or  spongy  texture;  so  that  it  is  common 
in  volcanic  ejectamenta  to  find  fragments  of  rock,  varying  in 
size  from  fine  dust  to  large  blocks,  in  which  a  cellular,  or 
pumiceous  structure  exists.  These  vesicles  are  sometimes 
coarse,  sometimes  so  fine  that  they  are  not  discernible  to 
the  naked  eye.  Most  of  the  fragments  of  rocks  and  crystals 
which  are  shot  up  from  the  crater,  fall  back  again,  unless 
there  be  a  sufficiently  strong  wind  blowing,  to  carry  them 
away.  The  constant  attrition  against  one  another  which 
they  undergo  during  these  repeated  journeys,  up  into  the  air, 
and  back  again  into  the  crater,  tends  to  round  off  any  angles 
which  the  fragments  may  possess;  and  the  process,  if  repeated 
long  enough,  would  reduce  the  whole  to  fine  sand  or  dust. 
Violent  explosions  also  affect  the  matter  within  the  flue  of 
the  volcano,  forming  a  large  amount  of  finely-comminuted 
and  dusty  material,  which  is  often  carried  by  the  wind  for 
long  distances,  or,  if  projected  in  calm  weather,  falls  in 
showers  over  the  cone.  In  some  of  the  high  volcanic  moun- 
tains in  the  Andes,  the  flow  of  lava  streams  over  the  snow 
and  ice,  which  rests  at  high  levels,  occasionally  causes  inun- 
dations, carrying  vast  quantities  of  fine  mud,  termed  moya,1 
composed  of  volcanic  dust  and  ashes ;  and  similar  mud-in- 

1  Dr.  Theodor  Wolf,  '  Der  Cotopaxi  und  seine  letzte  Eruption  am 
26  Juni,  1877,'  Neues  Jahrbuch  fur  Min.  u.  Geol.  Jahrgang  1878, 
Heft  ii.  p.  167. 


Volcanic  Ejectamenta.  267 

undations  are  also  produced  there  by  the  bursting  of  subter- 
ranean reservoirs  of  water  during  earthquakes.  *  Mud  derived 
from  this  source  descended,  in  1797,  from  the  sides  of  Tun- 
guragua  in  Quito,  and  filled  valleys  a  thousand  feet  wide  to 
the  depth  of  six  hundred  feet,  damming  up  rivers  and  causing 
lakes.  In  these  currents  and  lakes  of  moya,  thousands  of 
small  fish  are  sometimes  enveloped,  which,  according  to 
Humboldt,  have  lived  and  multiplied  in  subterranean  cavities.' * 

Volcanic  ashes  commonly  consist  of  small  fragments  of 
lavas,  and  crystals  of  felspars,  augite,  olivine,  biotite,  mag- 
netite, &c.,  and,  in  general,  there  is  a  more  or  less  close 
relation  in  the  minerals  which  constitute  volcanic  ashes  and 
sands,  and  the  mineral  constitution  of  the  lavas  which  have 
been  erupted  from  the  same  crater.  Volcanic  ashes  very  often 
contain  particles,  or  fused  drops,  of  vitreous  matter,  and  the 
crystals  which  occur  in  ashes  also  frequently  contain  nume- 
rous glass  inclosures.  The  plagioclase  crystals  which  occur 
in  the  ashes  of  Etna  are  especially  rich  in  glass  inciosures, 
but  the  plagioclase  in  the  Etna  lavas  also  contains  them  in 
great  quantity.2 

Volcanic  sand  simply  differs  from  ash  in  the  constituent 
fragments  being  coarser.  The  puzzolana  of  Naples  and  the 
gravier  noir  of  the  Puy  Gravenoire  in  Auvergne  are  volcanic 
sands,  used  in  the  manufacture  of  hydraulic  mortar. 

Lapilli  are  moderate-sized  fragments  of  rock,  usually 
scoriaceous  lava,  which  have  been  ejected  from  a  crater. 
They  may  either  occur  imbedded  in  deposits  of  ashes  and 
sand,  or  they  may,  of  themselves,  constitute  accumulations. 

The  ejected  lapilli  are  sometimes  pumice  fragments  and, 
at  times,  form  entire  volcanic  cones,  as  in  some  of  the  craters 
in  the  Lipari  Islands. 

Volcanic  bombs  vary  considerably  in  character,  but,  gene- 
rally-speaking, they  may  be  defined  as  masses  of  molten  rock- 

1  Lyell's  Principles  of  Geology,  Qth  edition,  p.  348. 

2  Etna,  a  History  of  the  Mountain  and  its  Eruptions,  by  G.   F, 
Rodwell,  p.  138.     London,  Kegan  Paul  &  Co.  1878. 


268  Descriptive  Petrology. 

matter,  which,  by  rotation  in  the  air.  during  their  upward 
flight  and  downward  fall,  have  assumed  a  more  or  less 
spherical  form,  and  have  wholly  or  partially  solidified  before 
again  reaching  the  earth ;  in  the  latter  case,  the  imperfectly- 
solidified  mass  sometimes  becomes  flattened,  by  impact  on 
the  surface  upon  which  it  falls.  Such  bodies  are  termed  slag- 
cakes. 

The  identification  of  very  old  deposits  of  volcanic  ash 
is  not  always  an  easy  task.  Where  numerous  lapilli  of 
scoriaceous  and  other  unquestionably  eruptive  rock  occur  in 
old  indurated  ashes,  as  in  those  of  Brent  Tor  in  Devon- 
shire, it  is  comparatively  easy  to  recognise  the  origin  of  the 
deposits ;  but  when  these  fail,  it  becomes  a  matter  of  con- 
siderable difficulty  to  say  with  any  certainty  whether  a  rock 
formed  of  broken  crystals,  such  as  might  characterise  any 
lava,  in  conjunction  with  very  finely  divided  matter,  such  as 
might  be  referred  either  to  fine  volcanic  dust  or  to  ordinary 
detrital  sediment,  really  represents  a  volcanic,  ash,  or  is 
simply  a  sediment  formed  wholly,  or  partly,  of  the  detritus 
of  pre-existing  eruptive  rocks.  Some  of  the  rocks  mapped 
as  ash  beds  in  the  English  Lake  district  have  undergone  a 
very  great  amount  of  alteration,  so  that  their  originally  frag- 
mentary character  is  only  revealed  by  superficial  weathering 
or  by  microscopic  examination,  and,  when  the  alteration 
becomes  extreme,  it  is  hardly  possible  to  distinguish  them 
from  compact  porphyritic  felsite.  Some  of  these  rocks 
closely  resemble  halleflinta,  and  a  determination  of  their 
precise  origin  is  a  difficult  exercise  for  micro-petrologists.  The 
recognition  of  ash  deposits  is  sometimes  rendered  trouble- 
some by  an  intimate  admixture  of  ordinary  sedimentary 
matter.  Much  yet  remains  to  be  done  in  the  determination 
of  old  volcanic  ejectamenta,  a  field  of  inquiry  in  which  none 
but  the  most  sceptic  are  likely  to  demonstrate  the  truth.1 

1  The  student  may  advantageously  consult  the  recent  paper  by  Dr. 
Albrecht  Penck,  '  Studien  iiber  lockere  vulkanische  Auswiirflinge. ' 
Zeitschr.  d.  Deutsch.  Geol.  Ges.  1878. 


Serpentine.  269 

ALTERED  ERUPTIVE  ROCKS. 

The  alterations  which  eruptive  rocks  undergo,  subsequently 
to  their  formation,  represent,  in  most  instances,  decomposition, 
often  accompanied  by  pseudomorphous  replacement  of  their 
constituent  minerals,  due  to  the  chemical  changes  effected  by 
infiltration  of  water,  charged,  either  with  carbonic  acid,  or 
carrying  in  solution  various  soluble  mineral  substances 
which  it  has  taken  up  during  its  passage  through  other 
rocks.  There  are  comparatively  few,  or  no  eruptive  rocks 
which  do  not,  to  some  extent,  show  traces  of  such  alteration, 
and  the  pseudomorphs  which  they  contain  are  so  numerous 
and  interesting  that  they  may  constitute  quite  a  special 
branch  of  study.  The  admirable  *  Recherches  sur  les 
Pseudomorphoses  '  by  Delesse,  indicate  how  much  may  be 
done,  and  yet  remains  to  be  done,  in  this  field  of  inquiry. 
Some  of  the  most  characteristic  pseudomorphs  will  be  found 
mentioned  in  the  descriptions  of  the  various  rocks  in  which 
they  occur. 

The  following  are  a  few  rocks  which  have  resulted  from 
the  decomposition  of  eruptive  rocks. 

Serpentine  has,  in  some  instances,  been  demonstrated  as 
the  result  of  the  decomposition  of  such  rocks  as  Iherzolite, 
gabbro,  &c.  It  is  also  quite  possible  that  serpentine  may 
sometimes  represent  the  alteration  of  ordinary  sedimentary 
rocks,  especially  magnesian  limestones,  as  suggested  by 
Jukes  and  other  geologists,  but  good  evidence  seems  as  yet 
to  be  wanting  upon  this  point.  Serpentine  is  also  stated  to 
result  from  the  decomposition  of  some  gneissic  rocks  and 
other  crystalline-schists,  also  from  garnet-rock  and  eklogite. 

Serpentine  is  a  fine-grained,  massive,  compact,  rather 
tough,  but  soft  rock,  of  very  variable  colour,  dark  and  light 
shades  of  green,  greenish-grey,  and  deep  red  being  the  most 
prevalent.  It  is  often  very  beautifully  veined  and  mottled 
with  other  colours,  and,  where  not  much  exposed  to  atmo- 
spheric influences,  it  forms  a  valuable  stone  for  decorative 


270  Descriptive  Petrology. 

work  in  architecture.  It  is  easily  turned  in  the  lathe  into 
columns  or  small  ornamental  articles,  and  takes  a  high 
polish.  It  often  contains  crystals  of  diallage,  which,  to  some 
extent,  add  to  the  beauty  of  the  stone.  Serpentine  is  also 
frequently  traversed  by  white  veins  of  steatite,  in  which 
angular  fragments  of  serpentine  are  sometimes  imbedded. 

Serpentine  is  essentially  a  hydrous  silicate  of  magnesia. 
When  pure  it  contains  at  least  two-thirds  of  silicate  of 
magnesia,  but  it  is  frequently  impure,  through  admixture  with 
silicate  of  protoxide  of  iron,  sesquioxide  of  chromium, 
argillaceous  matter,  and  carbonates  of  lime  and  magnesia. 
The  variations  in  its  colour  are  due  to  different  states  of 
oxidation  of  the  ferruginous  matter  which  the  rock  contains. 
Serpentine  when  heated  yields  on  an  average  about  12  per 
cent,  of  water. 

The  minerals  which  occur  as  accessories  in  serpentine  are 
very  numerous,  and  are  for  the  most  part  the  same  as  those 
which  are  met  with  as  accessories  in  the  crystalline  schists,  with 
which  rocks  serpentine  is  very  commonly  associated  and 
interbedded.  To  them,  however,  may  be  added  chromic  iron, 
picotite,  bronzite,  schiller-spar,  hematite,  both  massive  and 
as  specular  iron,  dolomite,  calcite,  brucite,  magnesite,  hydro- 
talcite,  native  copper,  copper  pyrites,  and  copper  glance. 
Gold  and  platinum  occur  in  the  serpentines  of  the  Ural. 

Serpentine  seems  especially  to  result  from  the  decompo- 
sition of  rocks  which  are  rich  in  olivine.  Professor  Bonney 
states  that  the  serpentine  of  the  Lizard  in  Cornwall  contains 
decomposed  olivine,  enstatite,  and  picotite,  and,  from  the 
presence  of  these  minerals,  he  regards  the  rock  as  altered 
Iherzolite,  similar  to  the  serpentine  into  which  he  had  pre- 
viously observed  the  typical  Iherzolite '  of  the  Ariege  to 
pass.  He  regards  the  Lizard  serpentine  as  a  truly  eruptive 
rock,  and  considers  that  the  sedimentary  rocks  which  sur- 
round it  had  been  metamorphosed  before  its  intrusion.1 

1  '  On  the  Serpentine  and  Associated  Rocks  of  the  Lizard  District,' 
T.  G.  Bonney,  Quart.  Journ.  Geol.  Soc.  vol.  xxxiii.  p.  923. 


Serpentine. 


271 


FIG. 


Sections  of  serpentine,  when  examined  by  ordinary  trans- 
mitted light,  usually  appear  of  a  pale  greenish  or  yellowish 
colour.  By  polarised  light  the  substance  commonly  exhibits 
a  more  or  less  fibrous  structure  which  displays  very  feeble 
polarisation,  pale  bluish-grey  and  neutral  tints  predominating. 
The  crystals  of  olivine,  when  they  are  only  partially  altered, 
appear  in  disconnected  fragments,  with  moderately  strong 
chromatic  polarisation,  the  spaces  between  the  fragments 
being  occupied  by  fibrous  serpentine,  which  represents  the 

incipient  decomposition  of  the 
olivine  along  those  irregular 
cracks  by  which  the  mineral  is 
so  frequently  traversed,  as  in 
fig.  88,  which  shows  part  of  a 
section  of  serpentine  from 
Coverack  Cove,  in  Cornwall, 
after  a  drawing  by  Professor 
Bonney,  The  speckled  por- 
tion of  the  figure  indicates  unaltered  olivine,  the  remainder 
serpentine.  Serpentine  frequently  contains  veins  of  a  finely 
fibrous  mineral,  chrysolite,  which  may  simply  be  regarded  as 
a  fibrous  condition  of  the  serpentine  itself. 

Occasionally  an  appearance  of  lamination,  or  fine  bedding, 
is  visible  on  the  weathered  surface  of  serpentine,  the  rock 
appearing  to  consist  of  thin  alternating  hard  and  soft  bands, 
but  the  cause  of  this  unequal  weathering  has  not  yet  been 
satisfactorily  determined. 

Serpentine  occurs  either  in  intrusive  bosses,  in  veins,  or 
in  beds  interstratified  with  gneiss,  mica-schist,  chlorite- 
schist,  talc-schist,  &C.1 

Potstone  is  a  soft,  sectile,  greenish-grey  rock,  composed 
of  chlorite,  talc,  and  serpentine,  used  in  Italy  for  the  manu- 
facture of  cooking-pots.  It  is  associated  with  serpentine 
and  chlorite-slate. 

1  The  views  entertained  by  Dr.  T.  Sterry  Hunt  on  the  origin  of 
serpentine  will  be  found  in  his  Chemical  and  Geological  Essays. 


272  Descriptive  Petrology. 

\Laterite  is  a  red,  earthy  rock,  which  occurs  in  beds  lying 
between  basalt  and  other  lava  flows,  and  results  from  their 
decomposition.)  It  is  strongly  impregnated  with  sesquioxide 
of  iron.  Hematite  and  beauxite  sometimes  occur  in  beds  of 
this  character.  (From  the  varying  nature  of  the  rocks  from 
which  it  is  derived,  laterite  has  naturally  a  very  variable 
composition,  and  indeed  there  is,  as  yet,  no  precise  defini- 
tion of  this  rock.) 

Palagonite-rock. — This  results  from  the  action  of  heated 
water  or  steam  upon  flows  of  lava,  which  effects  the  decom- 
position of  many  of  the  constituent  minerals,  and  especially 
causes  the  peroxidation  of  any  protoxide  of  iron  compounds 
which  the  rock  contains.  The  result  is  an  amorphous,  semi- 
vitreous  substance  of  extremely  variable  colour  (yellow,  red, 
brown,  and  black).  The  chemical  composition  of  palago- 
nite  corresponds  more  or  less  with  that  of  the  rock  from 
which  it  is  derived,  except  that  no  protoxide  of  iron  remains, 
as  it  is  all  converted  into  sesquioxide,  save  in  a  few  rare 
instances  where  magnetite  occurs.  The  qualitative  compo- 
sition of  the  rock  is  represented  by  silica,  alumina,  sesqui- 
oxide of  iron,  magnesia,  lime,  soda,  potash,  and  water.  The 
percentage  of  silica  mostly  ranges  between  30  and  40. 

Under  the  microscope,  palagonite  appears  as  a  perfectly 
amorphous  substance,  in  which  triclinic  felspars,  augite, 
olivine,  undetermined  microliths,  and  patches  of  colourless 
devitrified  matter  with  a  radiating  fibrous  structure  occur. 
These  last  show  dark  interference-crosses  in  polarised  light. 

Palagonite-tuffs  differ  from  palagonite  rock  in  consisting 
not  wholly  of  palagonite,  but  of  fragments  of  that  mineral, 
mixed  with  crystals  of  augite,  olivine,  and  fragments  of 
eruptive  rocks. 

Kaolin  or  China-clay  is  a  soft,  white,  earthy  rock,  which 
results  from  the  decomposition  of  the  felspar  in  granites. 
When  pure,  it  may  be  regarded  as  a  bisilicate  of  alumina, 
plus  two  equivalents  of  water;  but  the  composition  varies. 
It  may  also  result  from  the  decomposition  of  leucite, 


Kaolin.  273 

beryl,  &c.,  but  all  the  important  deposits  of  China-clay  are 
in  the  main  derived  from  orthoclase.  These  deposits  are 
sometimes  rather  impure  from  the  presence  of  other  consti- 
tuent minerals  of  granite.  Some  of  them,  in  which  quartz 
is  plentiful,  are  termed  China-stone. 

The  use  of  these  clays  for  the  manufacture  of  porcelain 
is  too  well  known  to  need  more  than  mention.  The  kaolin 
of  Cornwall  was  first  employed  for  this  purpose  by  William 
Cookworthy  of  Plymouth  in  1755.  It  has  to  be  carefully 
levigated  before  it'  is  fit  for  the  potteries. 


274  Descriptive  Petrology. 

SEDIMENTARY  ROCKS. 

The  general  character  of  sedimentary  rocks  has  already 
been  described  at  page  15  et  seqq.  In  this  place,  merely 
the  lithological  characters  and  industrial  applications  of  the 
most  typical  varieties  will  be  dealt  with.  These  rocks  con- 
stitute so  large  a  proportion  of  the  earth's  crust,  and  have 
such  an  important  bearing  upon  water-supply,  agriculture, 
and  mining  and  enginering  operations,  their  application  for 
building,  road-making,  and  other  industrial  purposes  is  so 
extensive,  and  the  history  of  their  formation,  and  of  the  past 
conditions  of  life  which  existed  at  the  time  of  their  depo- 
sition, as  shown  by  their  fossils,  presents  so  many  points  of 
scientific  interest,  that  it  would  be  impossible,  even  within  the 
limits  of  a  very  large  volume,  to  do  even  moderate  justice  to 
so  great  a  subject.  It  has  not  been  with  any  desire  to 
underrate  the  importance  of  the  sedimentary  rocks  that  so 
comparatively  large  a  proportion  of  this  work  has  been 
devoted  to  the  description  of  their  eruptive  brethren,  but, 
because  an  elementary  knowledge  of  their  mineral  constitu- 
tion and  structure  is,  when  compared  with  that  of  the 
eruptive  rocks,  far  more  simple  for  the  student  to  acquire. 

The  sedimentary  rocks,  as  already  stated,  may  be  divided 
into  two  series,  the  unaltered  or  normal,  and  the  altered  or 
metamorphic.  In  the  latter  series,  extreme  phases  of  alte- 
ration carry  the  metamorphic  rocks  out  of  the  sedimentary, 
and  into  the  eruptive  division.  These  eruptive  rocks,  when 
brought  to  the  surface,  and  subsequently  denuded,  supply  the 
materials  from  which  fresh  sediments  are  partly  formed,  so 
that  petrology  becomes  the  study  of  an  endless  cycle  of 
changes  from  eruptive  to  sedimentary,  and  from  sedimentary 
to  eruptive  rocks.  The  former  class  of  changes  are  the 
result  of  atmospheric  and  marine  denudation,  and  are 
due,  more  to  mechanical  than  to  chemical  agency.  The 
changes  of  the  latter  class  are  chemical  and  physical  in  their 
nature. 


Sedimentary  Rocks.  275 

The  altered  or  metamorphic  rocks  form,  therefore,  a 
transitional  series  between  the  unaltered,  or  normal-sedi- 
mentary, and  the  eruptive  series.  Sometimes  the  alteration 
is  so  slight  that  it  is  difficult  to  detect,  and  its  precise  nature 
still  more  difficult  to  demonstrate  ;  at  others,  where  great 
alteration  has  taken  place,  it  is  almost,  and,  in  extreme 
phases,  quite,  impossible  to  say  with  certainty  whether  a  rock 
should  be  referred  to  the  metamorphic  or  to  the  eruptive 
series,  since  there  is  no  natural  boundary  between  them. 

Before  considering  those  which  have  been  altered,  it  will 
be  better  to  describe  the 

UNALTERED  OR  NORMAL  SEDIMENTARY  SERIES. 

These  rocks  may  be  classed  as  arenaceous,  argillaceous, 
and  calcareous.  They  are,  however,  of  a  more  or  less  mixed 
character  as  a  rule,  the  arenaceous  rocks  being  often  ce- 
mented by  calcareous  matter,  the  argillaceous  and  calcareous 
rocks  frequently  containing  a  certain  admixture  of  sand  ; 
while,  again,  some  of  the  argillaceous  series  are  impregnated 
with  a  variable  amount  of  carbonate  of  lime,  and  those  of 
the  calcareous  series  are  sometimes  more  or  less  argillaceous. 
Some  of  the  normal  sedimentary  rocks  contain  fragments  of 
felspars,  scales  of  mica,  and  other  detritus,  derived  from  the 
disintegration  of  pre-existing  eruptive  rocks.  The  sedi- 
mentary rocks  occur  in  strata  or  beds,  which  rest  upon  one 
another,  which  have  a  regular  order  of  sequence,  and  which 
generally  contain  fossils  of  characteristic  types.  The  re- 
marks made  in  the  earlier  part  of  this  work,  and  the  nume- 
rous text-books  of  geology,  which  deal  more  or  less  fully 
with  the  stratigraphical  and  palaeontological  branches  of  the 
science,  render  it  unnecessary  to  say  anything  here  upon 
these  subjects. 

The  materials  of  which  sedimentary  rocks  consist  are 
usually  more  or  less  rounded  by  attrition,  the  result  of 
their  transport  by  water,  or,  in  the  case  of  aeolian  rocks,  of 
their  transport  by  wind. 

T  2 


276  Descriptive  Petrology. 

ARENACEOUS  GROUP. 
(Sandstones.} 

These  rocks  consist  essentially  of  grains  of  silica.     They 
either   occur  as  superficial  accumulations    of   loose    sand 
forming  desert  tracts,  or  low-lying  districts  on  sea  coasts, 
where  the  wind  piles  the   sand  up  in  dunes,  or  they  may 
occur  as  beds  of  loose  sand,  interstratified  with  coherent  beds 
of  rock.     They  are  also  met  with  in  a  state  of  more  or  less 
imperfect  consolidation,  the  grains  being  feebly  held  together 
by  an  iron-oxide  or  by  calcareous  matter  •  or  they  may  be 
excessively  hard  and  compact,  the  constituent  grains  being 
cemented  by  either  silica,  carbonate  of  lime,  iron-oxides  or 
carbonate  of  iron.     The  rocks  called  grits  vary  considerably 
in  lithological  character.     The  term  «  grit '  appears  indeed 
to  be  very  ill-defined.     The   millstone  grit,  which  may  be 
taken  as  one  of  the  leading  types,  is  more  or  less  coarse- 
grained, while   some    of    the    Silurian   rocks,  such   as  the 
Coniston  and  Denbighshire  grits,  are  frequently  very  fine- 
grained  and  compact  in  character.     Under  these  circum- 
stances it  seems  that  a  grit  may  best  be  defined  as  a  strongly- 
coherent,  well-cemented,  or  tough  sandstone,  usually,  but  not 
necessarily,  of  coarse  texture.    In  some  few  cases  there  even 
appears   to  be,  according  to   Prof.    Morris,   no  cementing 
matter  present,  as  in  some  of  the  new  red  sandstones,  the 
constituent  grains  being  apparently  held  together  merely  by 
surface  cohesion  superinduced  by  pressure.     It  is  not  pos- 
sible within  the  limits  of  this  work  to  do  more  than  allude  to 
some  of  the  most  important  sandstones  which  occur  in  the 
British  Isles.     Those  used  for  building-stone  and  paving  are 
for  the  most   part  of  old  red,  carboniferous,    triassic,  and 
neocomian  age.     Commencing  with  the  oldest  and  lowest  in 
the  series,  the  Cambrian  and  Silurian  grits  are  for  the  most 
part  very  tough,   closely  compacted  sandstones,  frequently 
containing    minute   fragments   of  felspars   and    sometimes 
scales   of  mica.     Their  constitution  implies  that  they  are 


Sandstones.  277 

formed,  at  all  events  to  some  extent,  from  the  detritus  of 
pre-existing  eruptive  rocks.  They  are,  in  some  instances, 
fusible  before  the  blowpipe,  on  the  edges  of  thin  splinters, 
which  is  probably  due  to  their  admixture  with  felspathic 
matter.  They  are  generally  traversed  by  numerous  joints, 
so  that  they  are  seldom  used  for  building  purposes,  except 
locally  in  the  construction  of  rough  walls.  They  are,  however, 
well  suited  for  road  metal,  and  in  some  places  good  flagstones 
are  quarried,  but  these  are,  for  the  most  part,  rather  to  be 
regarded  as  sandy  shales  and  slates,  than  true  sandstones. 
The  flaggy  sandstones  are  generally  micaceous,  and  to 
this  circumstance  their  fissile  character  is  often  due. 
Although  some  beds  of  sandstone  and  grit  occur  in  the 
Devonian  series,  they  are  unimportant  from  an  economic 
point  of  view,  but  in  the  Old  Red  Sandstone  (the  chrono- 
logical equivalent  of  the  Devonian  series),  both  building- 
stones  and  flagstones  are  quarried.  They  are  mainly  em- 
ployed in  the  districts  where  the  stone  is  procured.  It  is 
often  of  a  deep  reddish-brown  or  purple  colour  owing  to  the 
presence  of  peroxide  of  iron;  at  other  times  it  is  greyish, 
occasionally  with  a  greenish  tinge.  The  stone,  if  judiciously 
laid,  is  tolerably  durable,  but  in  some  old  buildings,  such  as 
Chepstow  Castle  and  Tintern  Abbey,  it  has  suffered  con- 
siderably from  the  weather.  Old  red  sandstone  is  extensively 
used  for  paving  in  many  of  the  large  towns  in  England, 
Scotland,  and  Ireland,  and  is  also  largely  employed  as  a 
general  building- stone  and  as  road  metal  in  the  districts 
where  it  is  quarried.  The  red  sandstone  quarries  of  Cork 
and  Kerry  yield  in  some  instances  building- stone  of  a  very 
durable  character.1  The  Dundee  and  Arbroath  sandstones, 
known  as  Caithness  flagstones,  quarried  on  the  east  coast  of 
Scotland,  form  good  and  durable  material  for  paving  and 
building,  but  the  former  is  too  sombre  in  colour  to  give  a 
pleasing  effect  when  used  for  architectural  purposes.2 

1  Building  and  Ornamental  Stones,  E.  Hull,  p.  266.     London,  1872. 

2  Applications  of  Geology  to  the  Arts  and  Manufactures,   Ansted, 
p.  153.      1865. 


278  Descriptive  Petrology. 

The  carboniferous  sandstones,  including  those  of  the 
Yoredale  series,  the  millstone  grit,  and  the  coal-measures, 
are  very  important  from  an  industrial  point  of  view,  since 
they  afford  good  material  for  building  and  paving.  The 
Halifax,  Bradford,  and  Rochdale  flags  are  extensively  used 
for  the  latter  purpose  in  the  North  of  England,  and  are  well 
known  to  builders  under  the  name  of  Yorkshire  flags.  Some 
of  them  absorb  water  very  readily,  consequently,  in  very 
exposed  and  damp  situations,  they  are  liable  to  flake,  espe- 
cially if  placed  in  positions  where  they  are  unable  to  part 
with  their  moisture.  The  stone  from  Bramley  Fall  near 
Leeds  belongs  to  the  millstone  grit,  and  is  largely  used  for 
architectural  purposes.  The  Rotherham  stone  is  worked  for 
building  purposes  and  for  grindstones,  and  that  at  Hart  Hill 
for  scythe-stones.  The  Wickersley  stone  (Middle  Coal- 
measure  Sandstone)  makes  good  grindstones. 

The  pennant  grits  and  sandstones  occurring  in  the  coal- 
measures  of  the  Bristol  coal-field  are  important  building- 
stones.  The  fine-grained  pale-brown  and  grey  sandstones 
from  Craigleith,  near  Edinburgh,  and  the  Binnie  quarry  in 
Linlithgowshire,  are  also  extensively  employed  for  buildings. 
They  darken  somewhat  on  exposure,  but  are  amongst  the 
most  durable  of  building-stones.  The  Craigleith  stone  con- 
tains only  about  i  per  cent,  of  carbonate  of  lime,  the 
cementing  medium  being  mainly  siliceous.  A  little  mica 
and  carbonaceous  matter  is  also  present  to  the  extent  of 
about  i  per  cent.,  the  remainder  of  the  rock,  98  per  cent., 
consisting  of  silica.  According  to  Ansted,  a  cubic  foot  of 
Craigleith  stone,  weighing  about  146  lbs.,will  absorb  4  pints 
of  water,  and  good  samples  will  resist  crushing  weights  to 
the  extent  of  5,800  Ibs.  to  the  square  inch.  Of  the  carboni- 
ferous sandstones  used  in  Ireland  the  Carlow  flags  are 
perhaps  the  most  important:  they  are  sometimes  more  or 
less  micaceous,  and  are  of  dark  bluish  or  grey  colour. 

The  Permian  sandstones,  which  are  the  equivalents  of 
the  Continental  Rothliegende,  or  lower  division  of  the  Per- 


mian  system,  are  but  little  used  in  this  country,  except 
locally,  for  building-stone,  as  in  some  parts  of  Cumberland,1 
Staffordshire,  Nottinghamshire,  and  Yorkshire.  At  Mans- 
field, in  Nottinghamshire,  reddish  brown  and  almost  white 
varieties  of  triassic  sandstone  are  quarried,  and  are  said  to 
be  durable.  As  a  rule  the  Permian  sandstones  are  not  well 
suited  for  building,  being  very  absorbent  and  liable  to  decay. 
The  Permian  sandstone  from  the  neighbourhood  of  St.  Bees 
was  used  for  the  construction  of  Furness  Abbey.  These 
rocks  have  mostly  a  deep  red  colour,  due  to  the  presence  of 
peroxide  of  iron,  which,  together  with  dolomitic  matter, 
constitutes  their  cement. 

Triassic  sandstones. — Those  belonging  to  the  upper 
trias  or  Keuper  are  the  most  important  as  building-stones, 
the  sandstones  of  the  lower  trias  or  Bunter  being,  as  a  rule, 
of  too  loosely-cemented  and  friable  a  character  for  such 
purposes.  The  latter  are,  however,  used  for  moulds  in 
foundries,  and,  occasionally,  for  buildings.  They  are  often 
variegated  and  mottled,  whence  the  name  Bunter — from  the 
German  bunt,  variegated  or  coloured—  and  they  frequently 
exhibit  false  bedding.  The  Keuper  series  affords  good 
building-stone,  especially  the  lower  Keuper  sandstones, 
which  are  extensively  used  in  the  midland  and  north-western 
counties.  It  is  of  pale  red,  brown,  and  yellow  colours, 
sometimes  almost  white,  and  is  mostly  fine-grained  and 
easy  to  work.  This  stone  has  been  largely  used  in  the 
cathedrals  of  Chester  and  Worcester. 

The  loosely-coherent  triassic  sandstone  of  Alderley  Edge, 
in  Cheshire,  is  partly  cemented  by  carbonates  of  copper  and 
other  mineral  matter,  derived  from  infiltrating  solutions.  To 
obtain  the  copper,  the  sandstone  is  crushed,  the  copper  salts 
dissolved  in  sulphuric  acid,  and  redeposited  on  scrap  iron  in 
the  metallic  condition.  The  sandstone  yields  but  little  more 
than  i  per  cent,  of  copper.  Keuper  sandstone  is  quarried 

1  The  Penrith  and  St.  Bees  sandstones  are  much  used  as  building- 
stones. 


28 o  Descriptive  Petrology. 

in  Antrim,  and  is  stated  by  Professor  Hull  to  be  exceedingly 
well  adapted  for  architectural  purposes. 

Amongst  the  carboniferous  and  triassic  rocks  of  some 
countries  a  sandstone  occurs  to  which  the  name  Arkose  is 
given.  It  consists  essentially  of  the  same  constituents  as 
granite,  and  has  been  derived  from  the  disintegration  of 
granitic  rocks.  Some  valuable  notes  upon  arkose  occur  in 
the  *  Me'moire  sur  les  Roches  dites  Plutoniennes  de  la 
Belgique  et  de  1'Ardenne  Frangaise/  by  MM.  Ch.  de  la 
Vallee  Poussin  and  A.  Renard,  Brussels,  1876,  p.  120. 

Jurassic  sandstones. — The  rocks  of  the  Jurassic  period 
are  for  the  most  part  limestones,  but  good  sandstone  is 
quarried  at  Aislaby,  near  Whitby,  in  Yorkshire,  and  has  been 
used  in  the  construction  of  Whitby  Abbey  and  several  other 
important  buildings.1  In  Lincolnshire,  Northamptonshire, 
and  Dorsetshire,  sandstone,  belonging  to  the  inferior  oolite, 
is  employed  for  building.  '  The  ferruginous,  or  calcareous 
rock  of  the  lower  part  of  the  Northampton  sand  is  locally 
largely  used  for  building  purposes,  but  it  does  not  usually 
possess  much  durability.  The  white  sands  in  the  upper 
part  of  the  series  are  extensively  dug  at  many  points  for 
making  mortar.'  2 

Cretaceous  sandstones. — Those  of  most  importance,  from 
an  economic  point  of  view,  are  the  sandstones  belonging  to 
the  Hastings  sand  series,  and  some  of  the  hard  sandstone 
beds  intercalated  with  the  Kentish  rag,  derived  from  the 
lower  greensand.  The  sand-rock  of  the  Hastings  Sands  is 
not  a  very  coherent  stone  when  first  dug,  but  it  hardens  on 
exposure,  and,  although  largely  used  for  building  in  the 
neighbourhood  where  it  is  quarried,  it  is  not  of  a  very  durable 
character.  It  is  generally  of  a  warm  yellowish  or  brownish 
colour,  and  has  a  somewhat  ferruginous  cement. 

Bargate   stone  is  quarried   at    Godalming   for   building 

1  'Mineral  Statistics,'  Mem.  Geol.  Sura.  part.  ii.  1858.     R.  Hunt. 

2  <  The  Geology  of  Rutland,'  J.  W.  Judd.     Mem.  Geol.  Swv.  p.  92. 
1875. 


Sandstones.  281 

purposes.  It  is  a  calcareous  sandstone,  and  occurs  in  the 
upper  part  of  the  Hythe  beds.  The  calcareous  sandstones 
in  the  Hythe  beds  in.  Kent  are  locally  termed  hassock,  and 
are  also  used  for  building.  The  Folkestone  beds  of  the 
lower  greensand  also  afford  hard  sandstone  and  grit,  suitable 
for  building  and  road-making.  In  the  upper  greensand  at 
Godstone  and  Merstham,  a  pale  calcareous  sandstone  called 
fire-stone  occurs,  which  is  well  suited  for  the  floors  of  fur- 
naces, and  is  also  a  durable  building-stone.1 

Flints  are  procured  from  the  upper  chalk,  and  are 
extensively  used  as  building  material  and  road-metal. 

Tertiary  sandstones. — Although,  in  this  country,  beds  of 
sand  are  of  constant  occurrence  in  the  tertiary  formations, 
they  are  not,  as  a  rule,  sufficiently  coherent  to  be  of  value 
for  building  purposes,  except  for  making  mortar.  They  are, 
when  pure,  used  in  the  manufacture  of  glass.  The  Headon 
Hill  sands,  which  occur  in  the  Bagshot  series  in  the  Isle  of 
Wight,  are  largely  used  for  this  purpose.  The  less  pure 
sands  are  applied  to  various  other  uses.  There  are,  how- 
ever, a  few  very  hard  tertiary  sandstones,  which  are  used  in 
this  country  for  building  and  paving,  some  of  which  are, 
according  to  Prof.  Morris,  derived  from  the  Woolwich 
series,  and  others  from  the  Bagshot  beds.  In  some  parts  of 
the  world  tertiary  sandstones  attain  great  importance.  Sand- 
stones of  miocene  age  constitute  a  considerable  part  of  the 
Himalayas.  As  an  appendix  to  the  rocks  of  this  group  we 
may  place  Tripoli,  a  fine  white  pulverulent,  chalk-like 
deposit,  which  consists  almost  exclusively  of  the  siliceous 
skeletons  of  diatoms.  To  demonstrate  this  it  is  only  need- 
ful to  wash  a  little  of  the  powder  on  to  a  slip  of  glass  and 
examine  it  under  the  microscope. 

The  mud  of  the  river  Parret  (which  runs  into  the  Bristol 
Channel)  affords  the  material  of  which  'Bath  bricks'  are 
made.  This  mud  has  been  stated  to  consist  almost  wholly 

1  See  also  'Geology  of  the  Weald,'  by  W,  Topley,  Mem.  Geol. 
Surv.  Eng.  6°  Wales i  p.  371. 


282  Descriptive  Petrology. 

of  the  remains  of  diatoms.  Microscopic  examination,  how- 
ever, disproves  the  statement,  and  shows  that  diatoms  form 
but  a  very  small  proportion  of  the  mud. 

ARGILLACEOUS  GROUP. 
(Clays,  Shales,  and  Slates.} 

These  rocks  are,  chemically  speaking,  impure  hydrous 
silicates  of  alumina.  Sometimes  the  impurity  consists  of 
sand,  sometimes  of  carbonate  of  lime  ;  and  more  or  less 
carbonaceous  matter  is  in  many  cases  present.  Their 
coarseness  of  texture  is  mainly  dependent  upon  the  coarse- 
ness of  the  sand  which  often  occurs  in  them.  When  free 
from  sand,  they  are  usually  of  fine  texture.  They  have  all 
originally  been  deposited  as  mud,  in  most  instances  at  the 
bottom  of  the  sea,  in  others  at  the  bottoms  of  lakes  or  as 
deltas,  and,  exceptionally,  over  land,  when  temporarily 
flooded  by  the  overflow  of  rivers,  as  in  the  case  of  the  Nile. 
Clay  deposits  often  have  a  well-laminated  structure,  and,  in 
the  older  geological  formations,  have  assumed  a  more  or  less 
indurated  character,  frequently  accompanied  by  a  tendency 
to  split  along  the  planes  of  bedding.  Very  often  another 
and  more  strongly-marked  fissile  structure  is  superinduced 
in  directions  cutting  across  the  planes  of  stratification  at 
various  angles.  This  is  slaty  cleavage,  described  at  page  35. 
Those  argillaceous  rocks  which  split  parallel  with  the  planes 
of  lamination  or  bedding  are  called  flags,  but  the  term  flag 
is  applied  to  a  rock  of  any  character  which  splits  along  its 
bedding  into  large  flat  slabs,  and  consequently  it  is  common 
to  find  the  term  used  to  denote  sandstones  which  are  suffi- 
ciently fissile,  when  quarried,  to  yield  slabs  or  flags.  To  the 
argillaceous  rocks  which  split  in  directions  other  than  that 
of  bedding  the  term  slate  is  given.  Still,  in  this  case,  the 
term  is  also  applied  to  rocks  which  differ  widely  from  ordi- 

1  '  Geology  of  East  Somerset  and  Bristol  Coal  Fields,'  Memoirs  of 
Gcol.  Surv.  p.  161.  H.  B.  Woodward,  1876. 


Clays,  S hales,  and  Slates.  283 

nary  slate.  The  Collyweston  slates,  calcareous  sandstones  of 
the  inferior  oolite,  and  the  green-slates  of  the  Lake  District, 
which  have  been  mapped  as  volcanic  ash  by  the  Geological 
Survey,  are  examples  of  the  application  of  the  term  slate 
as  indicative  of  fissile  structure,  and  not  of  lithological 
character. 

Cambrian  slates. — These  are  very  important  rocks, 
affording  compact  roofing-slates  of  admirable  quality,  mostly 
of  a  dark  purple  or  greenish  colour,  and  capable  of  being 
split  into  very  thin  and  large  slates,  exceedingly  free  from 
pyrites,  which  is  common  in  many  slates,  but,  from  its 
decomposition,  is  most  detrimental  to  them  as  roofing 
material.  The  slates  of  the  Penrhyn  and  Bangor  and  of 
the  Dinorwig  or  Llanberis  quarries  in  North  Wales  are  of 
Cambrian  age. 

Silurian  slates  and  flags. — The  Skiddaw  [lower  Silurian 
slates  of  Cumberland],  are  black,  or  dark-grey  rocks,  which 
are  often  traversed  by  many  sets  of  cleavage  planes,  causing 
them  to  break  up  into  splinters  or  dice,  so  that  no  good 
roofing-slate  can,  as  a  rule,  be  procured  from  them.  The 
best  lower  Silurian  slates  of  North  Wales  are  quarried  in 
the  Llandeilo  and  Bala  beds.  They  are  black,  dark  grey, 
and  pale  grey.  Ffestiniog,  Llangollen,  and  Aberdovey  are 
among  the  principal  quarries.  The  cleavage  in  these  rocks 
is  often  wonderfully  perfect  and  even,  so  that  occasionally 
slates  ten  feet  long,  six  inches  or  a  foot  wide,  and  scarcely 
thicker  than  a  stout  piece  of  cardboard,  are  procured. 
These  remarkably  thin  slates  are  tolerably  flexible.  The 
upper  Silurian  rocks  also  afford  good  slates  and  flags  in 
certain  localities,  while  the  rough  material  serves  for  local 
building  purposes.  In  many  parts  of  the  English  Lake 
District  the  houses  are  commonly  constructed  of  rough 
slates  and  flags  derived  from  the  Bannisdale  and  Coniston 
series.  The  quoins  of  the  better  class  of  these  houses  are 
often  built  of  a  light-coloured  freestone,  and  the  general 
effect  is  good,  although  sombre.  Silurian  slates  are  quarried 


284  Descriptive  Petrology. 

in  Scotland  in  Inverness-shire,  Perthshire,  and  Aberdeen- 
shire;  also  at  Killaloe  and  some  other  localities  in  Ire- 
land. 

Devonian  slates  of  a  grey  colour  are  worked  in  Cornwall, 
at  the  Delabole  and  Tintagel  quarries,  and  in  Devonshire, 
in  the  neighbourhood  of  Tavistock,  at  Wiveliscombe  and 
Treborough  in  Somersetshire,  and  in  other  parts  of  the 
United  Kingdom. 

The  carboniferous  flags  are  quarried  for  roofing  and 
paving  purposes  at  several  places  in  Yorkshire,  Lancashire, 
and  other  counties,  where  carboniferous  rocks  occur,  and 
are  mainly  procured  from  the  coal  measures.  They  are  of 
dark-grey  colour  or  black,  and  are  principally  used  in  the 
neighbourhoods  where  they  are  quarried. 

There  are  no  true  clay  slates  of  later  age  in  Great  Britain, 
but  in  other  parts  of  the  world  slates  of  even  tertiary  age 
occur. 

Of  the  clays,  used  in  this  country  for  economic  purposes, 
may  be  mentioned  the  china  clays  or  kaolins  of  Cornwall, 
which  have  been  formed  from  the  decomposition  of  the 
felspathic  constituents  of  granite ;  the  Watcombe  clay,  which 
occurs  in  the  trias,  and  is  now  used  in  the  manufacture  of 
terra-cotta  pottery  \  the  calcareous  liassic  clays,  used  for 
brick-making  and  burning  for  lime  and  hydraulic  cement ; 
the  various  clays  of  oolitic  and  neocomian  age,  some  of 
which  are  used  for  brick-making,  &c.  ;  the  gault,  the  clays 
of  the  Woolwich  and  Reading  beds,  and  the  London  clay, 
all  of  which  are  used  for  bricks  ;  the  celebrated  Poole  clay, 
dug  at  Wareham,  which  belongs  to  the  Bagshot  series,  and 
is  extensively  used  for  pottery.  The  clays  of  the  Bovey 
beds,  large  quantities  of  which  are  annually  shipped  at 
Teignmouth,  afford  good  pottery-clays  and  pipe-clays. 
There  are  also  many  brick-earths  and  clays  of  post- tertiary 
age  which  are  extensively  used  for  brick-making  and  other 
purposes.  Fuller's  Earth  is  a  yellowish,  greenish  or  bluish 
clayey  rock  containing  about  50  per  cent,  of  silica,  20  per 


Limestones.  285 

cent,  of  alumina,  25  per  cent,  of  water,  and  a  little  oxide  of 
iron.  It  chiefly  occurs  between  the  Inferior-  and  Great- 
Oolite,  and  in  the  Lower  Greensand.  The  river-mud  in  the 
Medway  and  at  the  mouth  of  the  Thames  is  largely  used  in 
the  manufacture  of  Portland  cement,  after  being  artificially 
mixed  with  chalk  and  burnt.  By  the  careful  levigation  of 
some  clays,  Dr.  John  Percy  has  eliminated  minute,  but 
beautifully- developed,  crystals  of  kaolinite. 

CALCAREOUS  GROUP. 
(Limestones.) 

These,  in  some  cases,  consist  almost  exclusively  of  car- 
bonates of  lime  and  magnesia  (magnesian-limestones  or 
dolomites),  while  occasionally  the}'  are  very  impure,  con- 
taining a  considerable  admixture  of  sand,  clay,  and,  in  some 
instances,  bituminous  matter.  The  British  Silurian  lime- 
stones are  of  comparatively  little  value,  except  for  lime, 
locally  employed  for  agricultural  purposes.  The  Devonian 
limestones  are,  however,  extensively  used  for  building  and 
paving,  and  some  of  them  are  well  adapted  for  ornamental 
purposes  on  account  of  the  richly  coloured  mottling  and 
veinings  which  they  frequently  exhibit.  The  carboniferous 
limestone  is  largely  used  for  building,  and  is  a  very  durable 
stone.  Some  of  the  highly  fossiliferous  beds — especially 
those  which  contain  numerous  fragments  of  encrinite  stems, 
locally  termed  screw-stones—constitute  handsome  marbles, 
the  fossils  being  white,  and  the  rock  itself  dark  grey,  or 
almost  black.  Good  encrinital  marble  is  quarried  at  Dent 
in  Yorkshire,  and  the  stone  is  much  used  for  chimney- 
pieces.  The  carboniferous  limestone  often  contains  bands 
and  nodules  of  chert.  The  magnesian  limestone  of  Per- 
mian age1  is  a  very  well  known  and,  when  judiciously  selected 
and  properly  laid,  a  very  durable  building-stone.  This  is 

1  Beds  of  magnesian  limestone  also  occur  in  the  carbonifeious  lime- 
stone series  in  Derbyshire  and  elsewhere. 


286  Descriptive  Petrology. 

well  shown  in  the  keep  of  Conisborough  Castle  and 
York  Minster,  which  have  both  been  built  of  Magnesian 
Limestone.  The  Museum  of  Practical  Geology  is  fronted 
with  this  stone,  and  has  stood  well.  The  Houses  of  Parlia- 
ment are  also  built  of  Magnesian  Limestone.  It  works  freely, 
and  can  generally  be  procured  in  large  blocks.  It  may 
here  be  observed  that  many  limestones  contain  only  a 
very  small  percentage  of  carbonate  of  magnesia,  and  since 
magnesia  and  lime  are  isomorphous,  the  amount  of  mag- 
nesian  carbonate  in  limestones  may  fluctuate  from  mere 
traces  to  a  ratio  of  CaCO3  to  MgCO3  =  1:3.  No  sharp 
line  of  demarcation  can  therefore  be  drawn  between  the 
dolomitic  limestones  and  the  true  dolomites,  in  which  the 
ratio  of  CaCO3  to  MgCO3  =  i  :  i  giving  the  percentage  com- 
position as  CaCO3=54'35,  MgCO3  =  45'65  for  normal 
dolomite. 

The  oolitic  limestones  are  so  numerous  and  constitute 
such  valuable  building-stones  that  it  is  only  possible  to 
mention  a  few  of  those  principally  employed.  These  are 
the  Doulting  stone,  belonging  to  the  inferior  oolite,  the 
Bath-stones  belonging  to  the  great  oolite,  of  which  the 
chief  kinds  used  are  the  Box  Hill  and  Corsham  Down 
stones.  The  Ketton  stone,  belonging  to  the  Lincolnshire 
oolites,  is  an  exceedingly  valuable  building-stone,  possessing 
great  tenacity,  working  freely,  and  resisting  atmospheric 
influences,  even  when  placed  in  unfavourable  situations. 
The  Ancaster  stone,  which  also  occurs  in  the  Lincolnshire 
oolites,  is  a  less  expensive  stone,  but  is  very  durable,  and 
is  extensively  used  for  building.  The  Portland  oolites 
afford  remarkably  good  stone,  which  is  used  very  largely, 
and  constitutes  one  of  the  most  important  building-stones 
in  this  country.  The  Purbeck  limestones,  which,  unlike 
the  preceding,  are  of  fresh-water  origin,  are  largely  used  for 
paving ;  while,  in  the  upper  part  of  the  series,  a  compact 
limestone,  crowded  with  fossil  shells,  of  the  genus  Paludina, 
is  known  as  Purbeck  marble,  and  has  been  used  for  small 


L  imes  tones.  287 

columns  and  other  architectural  decoration,  for  some  cen- 
turies. The  Petworth  marble  has  also  been  applied  to 
similar  purposes.  The  oolitic  limestones,  as  a  rule,  differ 
structurally  from  the  limestones  of  older  and  of  more  recent 
date,  inasmuch  as  that  they  are  usually  aggregates  of  little 
spherical  deposits  of  carbonate  of  lime,  which  have  formed 
in  concentric  crusts  around  nuclei.  These  nuclei  consist 
sometimes  of  a  granule  of  sand,  sometimes  of  the  remains 
of  a  minute  organism.  The  little  spherules  are  seldom 
much  bigger  than  a  large  pin's-head,  and  they  are  also 
cemented  together  by  calcareous  matter.  The  name  oolite 
is  derived  from  the  egg-like  or  fish-roe-like  appearance  of 
the  stone  ;  but  oolitic  structure,  although  characteristic  of 
the  limestones  of  oolitic  age,  is,  however,  not  exclusively 
peculiar  to  them,  for  well-developed  oolitic  structure  occurs 
in  certain  beds  of  the  carboniferous  limestone  near  Bristol, 
while  it  is  also  developed  in  the  coarser  pisolites  or  pea- 
travertines  of  recent  date.  The  older  limestones,  such  as 
those  of  the  Devonian,  carboniferous,  and  Permian  forma- 
tions, are  either  granular  or  crystalline-granular,  and  the 
latter  character  is  beautifully  shown  in  certain  limestones  at 
and  near  their  contact  with  eruptive  rocks.  The  saccharoid 
statuary  marbles  of  Italy  are  good  examples  of  this  struc- 
ture, and,  when  examined  in  thin  slices  under  the  micro- 
scope, are  seen  to  consist  of  closely  aggregated  crystalline 
grains,  in  each  of  which  polarised  light  reveals  the  existence 
of  numerous  twin  lamellae,  the  twinning  taking  place  along 
planes  parallel  to  the  face— JR.  It  seems  impossible,  in 
many  cases,  to  say  whether  this  structure  in  limestones  has 
been  due  to  the  metamorphism  engendered  by  the  contact 
or  proximity  of  eruptive  rocks,  or  whether  it  is  owing  to 
other  causes,  since  we  find  precisely  the  same  structure  in 
the  amygdaloids  of  calcspar  which  have  been  infiltered,  into 
the  vesicles  and  crevices  in  basalts,  long  after  their  solidifi- 
cation ;  we  find  it  in  the  fossils  of  the  chalk  and  of  other 
formations,  which  have  not  had  the  opportunity  of  becoming 


288  Descriptive  Petrology. 

altered  by  the  presence  of  intrusive  rocks ;  and  we  also  find 
it  in  limestones,  at  and  near  their  contact  with  eruptive 
masses,  as  already  observed.  The  earthy  variety  of  lime- 
stone, chalk,  has  been  stated  to  consist  exclusively  of  the 
calcareous  tests  of  foraminifera.  This,  however,  is  not 
always  the  case.  Samples  of  chalk  may  sometimes  be  care- 
fully levigated  and  examined,  and  foraminiferal  remains  may 
only  be  detected  here  and  there,  the  greater  part  of  the 
matter  having  merely  the  character  of  an  ordinary  amor- 
phous precipitate ;  while,  again,  other  samples  may  be 
found  to  consist  in  great  part  or  almost  entirely  of  the  re- 
mains of  these  organisms.  Some  writers  have  even  gone  so 
far  as  to  express  an  opinion  that  nearly  all  limestones  have 
been  formed  out  of  the  calcareous  remains  of  foraminifereti 
corals,  &c.  In  controversion  of  these  statements,  Credner, 
besides  giving  other  good  reasons,  appeals  to  microscopic 
evidence,  which  shows,  he  observes,1  'that  our  ordinary 
compact  limestones  are  by  no  means  always  formed  of 
broken  and  finely-ground  organic  remains,  but  rather  of 
little  rhombohedra  of  calcspar.'  On  the  other  hand,  how- 
ever, we  are  bound  to  admit  that  the  fossils  which  occur  so 
plentifully  in  limestones,  at  all  events,  represent  something 
more  than  an  insignificant  proportion  of  their  bulk,  and  in 
some  cases  seem  even  to  constitute  the  greater  part  of  the 
rock. 

Cretaceous  limestones. — The  Kentish  rags  are  mostly 
very  hard  sandy  limestones,  and  contain  more  or  less  dark- 
green  glauconite,  generally  in  fine,  occasionally  in  coarse, 
roundish  grains.  Glauconite  is  stated  to  sometimes  form 
the  cementing  medium  in  these  rocks,  but  more  or  less  car- 
bonate of  lime  is  always  present  in  this  capacity.  By 
decomposition,  the  protoxide  of  iron  in  the  glauconite  is 
converted  into  peroxide  of  iron,  and  the  rock,  under  these 
circumstances,  assumes  a  reddish-brown  tint.  According  to 

1  Elemente  der  Geologie,  Leipzig,  1876,  p.  290. 


Limestones.  289 

:nberg,  the  glauconite  grains  often  fill,  invest,  or  replace 
the  tests  of  foraminifera.  These  rocks  form  very  durable 
building-stones.  Besides  their  use  in  ashlar  work  they  are 
often  laid  in  irregularly-shaped  blocks,  giving  rise  to  a 
honeycomb  pattern  on  the  surfaces  of  the  walls  built  of  them. 
Kentish  rag  is  chiefly  quarried  at  Maidstone,  Hythe,  and 
Folkestone,  and  is  extensively  used  for  building  in  the  South 
of  England.  It  is  derived  from  the  Hythe  beds.1  Lime- 
stone, either  as  ordinary  chalk  or  as  subordinate  beds  of  com- 
pact limestone,  represents  a  considerable  part  of  the  creta- 
ceous series  of  rocks,  while  most  of  the  cretaceous  sandstones 
are  very  calcareous.  The  chalk  attains  a  great  thickness  in 
some  parts  of  the  kingdom ;  the  lower  portion,  termed  the 
grey  chalk  or  chalk  marl,  is  generally  slightly  glauconitic  at 
the  base.  The  upper  chalk  contains  numerous  nodules,  and 
occasionally  bands  of  flint,  which  follow  the  stratification, 
although  at  times  vertical  bands  of  flint  occur,  filling  up 
what  once  were  open  fissures.  Chalk,  besides  being  largely 
burnt  for  lime,  is  also  locally  used  for  building.  Certain 
hard  beds  occur  in  the  chalk  which  are  better  suited  for  this 
purpose  than  the  softer  material. 

Tertiary  limestones. — In  the  British  Isles  these  are  but 
poorly  represented.  The  Binstead  limestone,  occurring  in 
the  Bembridge  beds  in  the  Isle  of  Wight,  has,  however,  been 
extensively  quarried,  and  has  been  employed  in  the  con- 
struction of  some  of  our  early  churches.  In  other  parts  of 
the  world  tertiary  limestones  often  attain  great  thicknesses, 
and  constitute  important  building  stones.  The  pyramids, 
for  example,  are  built  of  nummulitic  limestone. 

There  are  many  other  interesting  tertiary  limestones, 
but  want  of  space  precludes  any  mention  of  them. 

ALTERED  SEDIMENTARY  SERIES. 
With  regard  to  the  rocks  of  this  series  it  is  difficult  to 

1  An  account  of  the  Wealden  marbles  will  be  found  in  Mr.  W. 
Topley's  'Geology  of  the  Weald,'  Mem.  Geol.  Surv.  Eng.  6°  Wales, 
p.  368. 

U 


290  Descriptive  Petrology. 

say  where  alteration  begins  and  where  it  ends,  still  more 
difficult  in  some  cases  to  define  the  nature  of  the  alteration. 
It  is  common  for  geologists  to  talk  about  altered  slates  where 
they  show  the  slightest  perceptible  difference  from  slates 
which  they  regard  as  normal  types,  but  it  is  often  open  to 
question  how  far  the  normal  types  are  really  normal,  and  to 
what  extent  the  microscopic  crystalline  constituents  of  these 
rocks  are  to  be  considered  normal  or  of  secondary  origin. 
Von  Lasaulx,  for  example,  states  a  belief  that  some  of  the 
microliths  in  slates  may  be  referred  to  hornblende  and 
epidote.  Now  the  latter  mineral,  if  present,  must  certainly 
be  regarded  as  a  secondary  product,  and  the  rock  which 
contains  it  must,  in  a  certain  sense,  be  considered  as  an 
altered  rock. 

The  rocks  of  the  altered  sedimentary  series  may  be 
divided  into 

A.  Those  with  no  apparent  crystallisation. 

B.  „          „     sporadic  crystallisation. 

C  Crystalline  \a'  ^on-foliated. 

(b.  foliated  and  schistose. 

Altered  sedimentary  Rocks  with  no  apparent  crystallisation. 
— In  the  case  of  limestones,  a  crystalline  or  crystalline- 
granular  condition  frequently  results  from  alteration,  but 
sometimes  the  change  simply  appears  to  cause  induration, 
without  developing  any  crystalline  structure,  as  in  some  of 
the  Antrim  chalk,  altered  by  the  proximity  of  basalt.  Sand- 
stones, from  the  character  of  their  constituent  particles,  can 
hardly  be  included  under  this  division  of  the  altered  sedi- 
ments. The  alteration  which  argillaceous  rocks  undergo 
without  begetting  any  perceptible  crystallisation  consists 
mainly  of  changes  which  appear  to  be  of  a  purely  physical 
character,  generally  slight  and  difficult  to  describe  intelli- 
gibly. Perhaps  the  best  example  of  such  alteration  is  to  be 
found  in  the  so-called  porcelain  jaspers,  clays,  or  shales, 
which  have  been  baked,  either  by  the  combustion  of  adjacent 


Chiastolite  Slate.  291 

coal-seams  or  by  the  contact  or  proximity  of  eruptive  rocks. 
Porcelain  jasper  has  a  fused  or  fritted  appearance,  a  slight 
gloss,  and  the  different  bands  or  laminae  often  assume 
strongly-marked  differences  of  colour,  in  which  dark  green 
and  brick-red  sometimes  predominate. 

Alte?'ed  sedimentary  Rocks  with  sporadic  crystallisation.-*- 
In  these  rocks  the  development  of  the  crystals  is  often  very 
imperfect  and  obscure  ;  in  some  cases,  however,  the  crystals 
are  distinct  and  well-developed. 

The  sporadic  crystals  which  occur  in  altered  limestones 
are  varieties  of  pyroxene,  usually  coccolite,  hornblende, 
garnet,  sphene,  tourmaline,  spinel,  phlogopite,  chlorite,  talc, 
&c.,  but  the  limestones  themselves,  in  which  these  crystals 
occur,  almost  invariably  have  a  crystalline  structure  en- 
gendered by  metamorphism,  and  consequently  they  should 
rather  be  placed  amongst  those  altered  sedimentary  rocks 
which  have  a  crystalline  structure. 

The  altered  slates  frequently  exhibit  sporadic  crystalli- 
sation. The  crystals  developed  in  them  are  usually  silicates 
of  alumina,  such  as  staurolite,  andalusite,  chiastolite,  &c. 

Chiastolite  slate  occurs  in  the  neighbourhood  of  granitic 
masses,  as  in  the  Skiddaw  district  in  Cumberland,  where, 
according  to  J.  Clifton  Ward,  '  on  approaching  the  altered 
area  the  slate  first  becomes  faintly  spotty,  the  spots  being  of 
a  somewhat  oblong  or  oval  form,  and  a  few  crystals  of  chias- 
tolite appear.  Then  these  crystals  become  more  numerous, 
so  as  to  entitle  the  rock  to  the  name  of  chiastolite  slate. 
This  passes  into  a  harder,  more  thickly-bedded,  foliated  and 
massive  rock,  spotted  (or  andalusite)  schist ;  and  this  again 
into  mica  schist  of  a  generally  grey  or  brown  colour,  and 
occurring  immediately  around  the  granite/ !  The  chiasto- 
Tite  slates  are  mostly  of  dark  grey  or  bluish-black  colour,  and 
contain  pale  yellowish-white  crystals  of  chiastolite,  some- 
times more  than  half  an  inch  in  length.  These  in  trans- 

1  'Geology  of  the  Northern  Part  of  the  English  Lake  District.' 
Memoirs  of  the  Geol.  Surv.  England  and  Wales  >  1876,  p.  9. 

U  2 


292  Descriptive  Petrology. 

verse  section  often  show  a  dark  central  spot  and,  occasion- 
ally, the  chiasmal  interpositions  which  characterise  this 
mineral. 

Staurolite  slate  is  a  dark  micaceous  slate  containing 
crystals  of  staurolite  ;  in  some  localities  passages  have  been 
observed  from  this  rock  into  andalusite  slates. 

In  some  altered  slates  the  staurolite  is  so  imperfectly 
developed  that  it  merely  appears  in  roundish  or  lenticular 
knots  or  lumps,  which  exhibit  no  approximation  to  crystal- 
line form. 

The  knoten-  frucht-  garben-  and  fleckschiefer  of  Ger- 
man petrologists  consist  of  micaceous  slates  containing 
small  irregular  concretions  or  little  lenticular  or  ovoid  bodies, 
which,  in  some  cases,  may  be  referred  to  andalusite,  but  in 
many  instances  they  are  shown  by  microscopic  examination 
to  be  aggregates  of  small  scales  of  mica,  carbonaceous 
matter,  quartz  granules,  and  other  constituents  of  the  rocks 
in  which  they  occur.  They  are  often  surrounded  by  ferru- 
ginous stains  resulting  from  decomposition.  The  deter- 
mination of  the  precise  character  of  these  bodies  is  often  a 
matter  of  considerable  difficulty. 

Want  of  space  precludes  any  description  of  other  inter- 
esting rocks  which  belong  to  this  group. 

ALTERED  SEDIMENTARY  ROCKS  (CRYSTALLINE). 

These  rocks  may  be  divided  into  (A]  non-foliated,  and 
(B)  foliated  and  schistose  groups. 

A.  Non-foliated  Group. 

Under  this  head  come  the  limestones  in  which  a  crystal- 
line structure  has  been  superinduced  by  the  proximity  of 
eruptive  rocks.  This  structure  differs,  in  no  essential  respect, 
from  that  of  the  crystalline  limestones  described  at  page  284. 
The  sporadic  crystals,  which  sometimes  occur  in  these  lime- 
stones, are  coccolite,  tremolite,  tourmaline,  sphene,  chondro- 


Quart zite.     Lydian-stone.  293 

dite,  spinel,  garnet,  mica,  chlorite,  &c.  Those  crystalline 
limestones  which  are  suitable  for  ornamental  architecture 
are  termed  marbles,  and  many  marbles  are  rocks  of  this 
kind,  which  owe  their  crystalline  character  to  alteration  by 
intrusive  masses ;  still  there  are  also  many  in  which  the 
crystalline  structure  is  not  due  to  this  cause.  The  term 
marble  is,  however,  very  loosely  employed,  and  may  be 
generally  taken  to  signify  any  rock  which  takes  a  good 
polish  and  is  employed  for  decorative  or  architectural  pur- 
poses. It  is  impossible,  from  want  of  space,  to  allude  even 
to  the  most  important  marbles. l 

Quartzite  is  a  compact  crystalline-granular  aggregate  of 
quartz,  either  in  irregular  crystalline  grains,  or  in  well- 
developed  crystals.  Some  quartzit.es  exhibit  a  schistose 
structure,  which  is  partly,  or  wholly,  due  to  the  presence  of 
small  quantities  of  mica,  the  scales  of  mica  lying  in  the 
direction  of  the  fissile  planes  in  the  rock.  These  schistose 
quartzites  may  therefore  be  regarded  as  mica  schists  poor  in 
mica. 

Lydian-stone  (basanite,  touch-stone,  kieselschiefer)  is  a 
dark-coloured,  generally  velvet-black  or  brownish-black 
rock.  It  is  an  altered  sandy  slate.  Under  the  microscope, 
it  is  seen,  in  great  part,  to  consist  of  crystalline  grains  of 
quartz  mixed  with  particles  of  argillaceous,  carbonaceous, 
and  ferruginous  matter.  The  percentage  of  carbonaceous 
matter  is  sometimes  considerable,  and  accounts  for  the 
extremely  black  colour  of  the  rock.  Lydian-stone  is  often 
traversed  by  small  veins  of  crystalline  quartz,  and  frequently 
contains  a  little  pyrites. 

B.  Foliated  and  Schistose  Group. 
These   rocks  which  are   commonly  designated   crystal- 

1  Good  accounts  of  the  Italian  marbles  will  be  found  in  the  Official 
Catalogue  of  the  Exhibition  of  1862,  '  Kingdom  of  Italy,'  section  v.  p. 
44 ;  and  in  The  Mineral  Resources  of  Central  Italy,  by  W.  P.  Jervis. 
London,  1868. 


294  Descriptive  Petrology. 

line  schists,  afford,  perhaps,  the  best-defined  instances  of 
metamorphism,  in  the  sense  in  which  that  term  is  usually 
applied.  They  are  generally  characterised  by  the  presence 
of  one  of  the  following  minerals,  hornblende,  mica,  chlorite, 
talc,  and  occasionally  schorl ;  but  gneiss,  described  under  the 
granitic  rocks  at  p.  2 TO,  has  the  same  constituents  as 
granite,  and  is  in  many,  if  not  in  nearly  all,  instances,  also 
an  altered  sedimentary  rock.  How  far  its  foliation  always 
represents  bedding,  is,  however,  a  point  which  does  not  yet 
appear  to  be  fully  demonstrated.  The  term  gneiss  has  been 
applied  to  many  rocks  which  have  not  the  same  mineral 
constitution  as  granite,  and  which  should  rather  be  referred 
to  hornblendic,  and  other,  schists,  into  which,  however,  gneiss 
sometimes  passes.  There  are  many  varieties  of  gneiss,  of 
which  the  most  important  were  alluded  to  under  the  gra- 
nitic rocks  at  page  210,  more  on  account  of  their  mineral 
constitution  than  of  their  origin,  mode  of  occurrence,  and 
structure,  which  latter  would  entitle  them  to  be  placed  in 
this  group. 

Gneiss1  is  a  foliated  crystalline  aggregate  of  the  same 
minerals  which  constitute  the  different  varieties  of  granite, 
typically,  of  orthoclase,  plagioclase,  quartz,  and  mica.  These 
minerals  are  arranged  in  more  or  less  distinct  layers  or  foliae 
which  are  approximately  parallel  to  one  another.  The  mica, 
especially,  forms  very  distinct,  although  thin,  bands,  and  it  is 
to  this  arrangement  of  the  mica  that  the  schistose  and  often 
fissile  character  of  the  rock  is  due.  Sometimes  the  mica  is 
a  potash,  sometimes  a  magnesian  mica,  and,  at  others,  both 
kinds  are  present.  Gneiss  varies  in  colour,  the  orthoclase  in 
some  varieties  being  red,  while  in  others  it  is  white  or 
greyish.  These  different  rocks  are  on  this  account  desig- 
nated red  gneiss  and  grey  gneiss,  and  it  has  been  shown,  by 
analysis  of  some  of  the  most  typical  examples,  which  occur 
in  the  neighbourhood  of  Freiberg,  that  there  is  a  marked 
chemical  difference  between  them,  the  red  gneiss  containing 

1  See  also  page  211. 


Gneiss.     Granulite.  295 

from  75  to  76  per  cent  of  silica,  while  the  grey  variety  pos- 
sesses only  from  65  to  66.  From  these  analyses  the  per- 
centage of  the  constituent  minerals  in  the  two  rocks  have 
been  deduced  as  follows  : 

Red  gneiss  :  orthoclase  =  60,  quartz  =  30  mica  =  10  per  cent. 
Grey  gneiss:          „         =45,      „      =25     „      =30        „ 

The  above  .  represent  extremes  of  variation,  while 
numerous  transitional  conditions  of  gneiss  exist  between 
them. 

In  some  varieties  of  gneiss  the  mica,  instead  of  lying  in 
parallel  foliae,  wraps  round  lenticular  aggregates  of  felspar 
and  quartz,  or  round  crystals  of  orthoclase.  These  varieties 
are  termed  pseudo-porphyritic  gneiss,  eye  gneiss  (augen 
gneiss],  wood  gneiss,  &c. 

Oligoclase,  dichroite,  garnet,  micaceous  iron,  magnetite, 
and  chlorite  sometimes  occur  plentifully  in  gneiss,  so  that 
they  impart  a  distinct  character  to  the  rock,  which  is  then 
accordingly  termed  dichroite  gneiss,  magnetite  gneiss,  &c. 

Protogine  gneiss. — This  rock  has  already  been  described 
at  page  212. 

Syenitic  or  hornblendic  gneiss  has  the  same  mineral 
constitution  as  syenitic  granite.  The  felspar  is,  in  great 
part,  represented  by  oligoclase.  It  is  a  rock  of  very  exten- 
sive occurrence,  and  passages  have  been  observed  from 
hornblende  gneiss1  into  hornblende  schist. 

Granulite1  or  leptinite  is  a  schistose  rock  composed  of 
orthoclase  and  quartz,  and  contains,  as  a  rule,  numerous 
small  garnets.  When  mica  is  present  the  rock  assumes  a 
more  or  less  schistose  structure,  and  passes  over  into  gneiss- 
granulite,  or  gneiss.  Schorl  is  of  common  occurrence  in  this 
rock.  The  margins  of  granitic  masses  sometimes  approxi- 
mate very  closely  in  character  to,  or  are  even  identical,  in 
mineral  constitution,  with,  granulite.  When  this  is  the 
case  the  rock  must  certainly  be  eruptive,  unless  it  can  be 

1  See  also  page  210. 


296  Descriptive  Petrology. 


regarded  as  an  alteration  of  adjacent  sedimentary  rocks.  At 
Brazen  Tor,  on  the  western  margin  of  Dartmoor,  the  granite 
assumes  quite  a  granulitic  character,  so  far  as  its  constituents 
are  concerned,  although  it  shows  no  foliation  or  schistose 
structure,  while  the  eruptive  rock  which  intervenes  between 
it  and  the  sedimentary  rocks  seems  to  preclude  the  idea 
that  its  origin  is  other  than  eruptive,  and  that  it  is  anything 
more  than  a  phase  of  the  granite.1  The  schistose  varieties 
of  granulite  must,  however,  be  regarded  as  altered  sedi- 
mentary rocks. 

Brian  rock  occurs  in  mica-slate  at  Erlhammer,  near 
Schwarzenberg,  in  Saxony.  It  is  a  fine-grained  or  compact 
aggregate  of  garnet,  albite,  and  quartz,  and  occupies  a  place 
intermediate  between  granulite  and  garnet  rock. 

Porphyroid. — Under  this  name  are  included  certain 
altered  sedimentary  schistose  rocks,  consisting  of  a  fine- 
grained matrix  of  felspar  and  quartz,  and  containing  a  large 
quantity  of  a  sericite-like  or  micaceous  mineral.  Within 
this  matrix  lie  numerous  crystals  of  felspar  (orthoclase  or 
albit£)  and  rounded  grains  or  crystals  of  quartz.  The  por- 
phyroids  occur  interbedded  with  other  old  sedimentary 
rocks. 

Seriate- Schist. — This  is  a  schistose  rock  closely  allied  to 
the  porphyroids,  and  consists  of  sericite,  fragments  of  quartz, 
albite,  and  usually  more  or  less  chlorite  and  mica.  There 
are  two  varieties,  the  green  and  the  red,  which  differ  some- 
what in  composition,  the  former  containing  much  albite 
and  little  or  no  mica,  while  the  latter  contains  mica  and 
very  little  albite.  In  some  cases  the  rock  is  rich,  in  others 
poor,  in  quartz.  The  very  fine-grained,  compact  varieties 
in  which  the  constituent  minerals  cannot  be  distinguished 
by  the  naked  eye,  are  termed  sericite-slates  or  sericite-phyl- 
lites.  Occasionally  augite  may  be  detected  in  some  varieties 
of  this  rock,  which,  in  such  cases,  are  evidently  allied  to  the 
diabase-schists  or  tuffs. 

1  '  The  Eruptive  Rocks  of  Brent  Tor,'  Memoirs  of  Geol.  Surv.  1878. 


Mica-Schist.     Chlorite- Schist.  297 

Mica-Schist  is  an  aggregate  of  mica  and  quartz.  The 
relative  amounts  of  these  two  minerals  varies  considerably, 
some  varieties  of  the  rock  consisting  almost  wholly  of  mica, 
while  others  are  composed  almost  wholly  of  quartz,  and 
contain  only  a  very  small  proportion  of  mica.  The  latter 
approximate  and  pass  into  quartz-schist.  All  of  these  rocks 
have  a  schistose  structure  due  to  the  parallel  arrangement 
of  the  crystals  and  scales  of  mica.  The  mica  in  these 
schists  is  sometimes  silvery-white  potash-mica,  sometimes 
dark  magnesian-mica,  but  the  former  is  by  far  the  most 
common.  The  quartz  is  in  small  grains,  often  of  a  flattened 
or  lenticular  form,  and  it  is  very  usual  for  the  quartz  and 
mica  to  constitute  alternating  parallel  layers,  so  that  the  rock 
exhibits  more  or  less  distinct  foliation.  The  percentage  of 
silica  in  mica-schists  varies  according  to  the  amount  of 
quartz  which  is  present,  the  extremes  of  fluctuation  being 
between  40  and  a  little  over  80  per  cent.  Garnets  are  of 
common  occurrence  in  these  rocks,  and  numerous  other 
minerals  are  met  with  as  accessories,  such  as  tourmaline, 
hornblende,  kyanite,  staurolite,  felspars,  epidote,  chlorite, 
talc,  magnetite,  pyrites,  specular-iron,  gold,  and  graphite. 
When  certain  of  these  minerals  preponderate,  the  rocks  pass 
over  into  schists  of  a  different  character,  to  which  special 
names  are  given.  In  some  of  these  rocks  calcspar  or  dolo- 
mite partly  or  wholly  replaces  the  quartz  and  films  of  argil- 
laceous matter;  chlorite  and  sericite  are  also  present  at 
times.  Such  rocks  are  known  as  calcareous  mica-schist, 
calcareous  chlorite-schist,  &c. 

Itacolumite  is  a  somewhat  schistose,  micaceous  sandstone 
consisting  of  granules  of  quartz  and  scales  of  mica,  talc, 
chlorite,  &c.  The  rock  is  flexible,  when  in  thin  slabs.  It 
occurs  in  N.  America  and  Brazil,  and  contains,  amongst 
other  minerals,  gold  and  diamonds. 

Chlorite- Schist  usually  occurs  interbedded  with  gneiss 
and  with  other  metamorphic  rocks.  It  is  of  a  green  or 
greenish-grey  colour,  and,  as  a  rule,  consists  in  great  part  of 


298  Descriptive  Petrology. 

scales  of  mica  closely  matted  together.  Quartz,  however, 
is  usually  present,  and  felspars,  mica,  and  talc  are  also  of 
common  occurrence.  Sometimes  it  exhibits  a  fine-grained 
and  evenly- cleavable  slaty  character  (chlorite-slate),  but  the 
coarser  schistose  varieties  usually  have  a  somewhat  imperfect, 
or  irregular  and  wavy,  fissile  structure.  The  chlorite-schists 
are  frequently  rich  in  accessory  minerals,  among  which  may 
be  cited  mica,  hornblende,  actinolite,  schorl,  epidote,  sphene, 
corundum,  garnet,  rutile,  specular-iron,  magnetite,  pyrites, 
copper-pyrites,  and  gold.  Passages  occur  from  chlorite- 
schist  into  mica-,  talc-,  and  hornblende-schists,  and  occa- 
sionally into  rocks  of  a  serpentinous  character. 

Talc-Schist  consists  of  scales  of  talc  with  a  small  admix- 
ture of  quartz.  It  is  a  greenish,  greyish- white,  or  yellowish- 
white  rock,  very  soft,  and  has  a  smooth  greasy  feeling  when 
rubbed  with  the  fingers.  Chlorite  and  mica  are  often 
present,  and  sometimes  a  little  felspar.  The  rock  contains 
as  accessories  many  of  the  minerals  which  occur  in  chlorite  - 
schist,  in  addition  to  which  may  be  mentioned  asbestus, 
hydrargillite,  fahlunite,  gahnite,  chlorospinel,  &c. 

Schorl-Schist  is  mainly  composed  of  schorl  and  quartz, 
and  often  shows  well-marked  foliation,  the  greyish  crystal- 
line-granular rock  being  traversed  by  approximately  parallel 
thin  black  bands  of  schorl  which  are  often  more  or  less 
contorted.  Cassiterite  very  commonly  occurs  in  this  rock. 
Mica,  chlorite,  felspars,  topaz,  and  arsenical  pyrites  occur  as 
accessory  constituents. 

Schorl  rock  is  a  granular,  non-foliated  rock  of  similar 
mineral  constitution. 

Amphibolite-  or  Hornblende- Schist  consists  of  hornblende 
and  quartz.  The  latter  mineral  is  often  present  only  in 
small  quantities.  It  is  a  dark  greenish-grey  or  iron-grey  rock, 
and  when  quartz  is  absent,  and  it  consists  exclusively  or 
almost  exclusively  of  hornblende,  it  is  then  termed  amphi- 
bolite.  Some  of  these  rocks  are  rather  difficult  to  identify 
without  microscopic  examination.  The  hornblende  is  some- 


Coarse  Fragment al  Rocks.  //   ,299 

times  granular,  at  others  in  radiating  prisms  or  iih/  a  fibrous* 
condition.     Magnetite  is  common  as  an-' accessory ,' drid  fel- 
spars, garnet,  biotite,  epidote,  and  pyrites  arWaJsO  .met  with/,  , 
although  they  are  always  subordinate  constituents.'    *  / 

Actinolite- Schist  may  simply  be  regarded  as  a  variety 
of  hornblende-schist.  The  constituents  are  actinolite  and 
quartz.  Felspars  are  occasionally  present  in  amphibolite. 
These  are  sometimes  triclinic,  and  the  rock  then  approxi- 
mates to  diorite. 

COARSE  FRAGMENTAL  ROCKS. 

These  may  be  divided  into  Breccias  and  Conglomerates. 
They  consist  of  materials  derived  from  the  waste  of  various 
rocks  and  are  made  up  of  fragments  either  angular,  or  sub- 
angular,  or  of  rounded,  waterworn  pebbles  or  boulders. 
Similar  but  much  finer  material  constitutes  many  of  the 
sedimentary  rocks,  and  it  is  merely  in  the  size  of  the  frag- 
ments that  many  sandstones,  grits,  &c.,  differ  from  breccias 
and  conglomerates,  the  former  being  as  much  entitled  to  be 
placed  among  the  fragmentary  or  clastic  rocks  as  the  latter. 
This  view  is  very  clearly  represented  by  Naumann's  classifi- 
cation of  the  clastic  rocks,1  which  he  divides  into  the 
psephitic  (from  ^r]0oe,  a  small  stone) ;  the  psammitic  (from 
^o'/z^oc,  sand) ;  and  the  pelitic  (from  Trr/Xde,  mud).  The 
psammites  and  pelites  of  the  two  last  groups  are  respectively 
represented  by  the  various  sandstones,  arkose,  &c.,  and  by 
the  tuffs  which  have  already  been  described  in  conjunction 
with  the  rocks  from  which  they  have  been  derived  ;  so  that 
it  only  remains  to  describe  the  breccias  and  conglomerates 
which  constitute  the  psephitic  division  of  the  clastic  rocks, 
although  the  coarse  materials  of  the  latter  are  often  mixed 
with  or  cemented  by  psammitic  and  pelitic  matter. 

BRECCIAS. 

The  breccias  differ  from  the  conglomerates  in  consisting 
1  So  named  from  K\acrT6s,  broken. 


300  Descriptive  Petrology. 

of  angular  or  sub-angular  fragments,  instead  of  rounded 
water- worn  pebbles  or  boulders ;  but  it  often  happens  that 
these  coarse  clastic  rocks  have  a  mixed  character ;  consisting 
partly  of  angular  and  coniparatively  unwater-worn,  and 
partly  of  rounded  water- worn  materials.  To  such  rocks  the 
name  breccio-conglomerate  is  sometimes  given.  The  frag- 
ments of  which  breccias  are  composed  are  usually  large 
enough  to  permit  the  recognition  of  their  lithological  cha- 
racter. They  are  often  derived  from  various  sources,  frag- 
ments of  sandstone,  quartz,  jasper,  and  various  eruptive 
rocks  being  common,  while  occasionally  they  consist  of  lime- 
stone fragments,  or  of  broken  pieces  of  bone,  as  in  some  of 
the  bone-breccias  which  occur  in  cave-deposits.  The  gene- 
rally angular  character  of  breccias  indicates  that  they 
have  been  formed  at  or  near  the  spots  where  they  occur  and 
that  their  materials  have  never '  travelled  far.  They  may  be 
formed  from  the  superficial  disintegration  of  rocks  in  their 
immediate  vicinity ;  they  may  represent  talus  and  rubbish 
heaps  which  have  been  subsequently  cemented  by  the  infiltra- 
tion of  calcareous,  siliceous,  or  ferruginous  matter ;  they  may 
result  from  the  breaking  in  or  '  creep '  of  rocks  with  which 
soluble  deposits  have  been  interstratified  and  subsequently 
dissolved  out,  as  in  the  case  of  the  haselgebirge,  occurring  in 
the  Northern  Alps,  where,  from  the  removal  of  underlying 
salt  beds,  a  breccia  has  been  formed,  consisting  of  fragments 
of  various  rocks  imbedded  in  a  matrix  of  clay. 

Breccias  may  also  result  from  the  breaking-away  of  small 
fragments  from  the  sides  of  fissures  (friction  breccias) 
through  which  dykes  of  eruptive  rock  have  been  subse- 
quently injected,  or  into  which  mineral  matter  in  solution 
may  have  subsequently  filtered.  The  latter  condition  is  a 
very  common  one,  and  may  be  well  observed  in  some  metal- 
liferous lodes.  In  such  cases  it  is  not  unusual  to  find  the 
fragments  enveloped  in  successive  deposits  of  different 
mineral  character.  The  accumulations  of  angular  and  sub- 
angular  ice-worn  and  scratched  stones  which  constitute  the 


Conglomerates.  301 

moraines  of  glaciers,  and  the  angular  unworn  rubbish 
dropped  by  the  melting  of  icebergs,  may  also  give  rise  to 
deposits  which  may  be  regarded  as  unconsolidated  breccias. 
Sometimes,  also,  they  are  formed  at  the  bottom  of  lava-flows, 
the  once  molten  rock  having  caught  up  and  enveloped  frag- 
ments of  various  kinds  and  sizes ;  and,  finally,  coarse  volcanic 
ejectamenta,  or  lapilli,  may  also  constitute  rocks  which  may 
justly  be  described  as  volcanic  breccias  or  volcanic  agglo- 
merates. 

Some  breccias,  when  polished,  are  well  adapted  for 
ornamental  purposes.  They  occur  in  formations  of  very 
different  ages,  and  they  have  an  especial  geological  interest 
as  indicating  either  the  local  disintegration  of  older  rocks, 
or  the  transport  of  materials  from  distant  countries  by  glacial 
agency. 

CONGLOMERATES. 

These  are  composed  of  pebbles  or  boulders  which  have 
been  either  carried  by  rivers,  or  washed  about  on  shores,  and 
consequently  rounded  by  attrition.  The  process  of  round- 
ing is  constantly  going  on  in  the  beds  of  our  rivers  and 
along  our  sea-coasts ;  and  the  beaches  which  are  formed 
along  our  shores,  would,  if  cemented,  become  true  conglo- 
merates. The  materials  composing  conglomerates  are,  like 
those  of  breccias,  of  very  variable  character,  being  derived 
from  many  different  sources,  but  they  are  mostly  formed 
from  rocks  of  considerable  hardness,  since  softer  fragments 
become  totally  disintegrated  by  constant  trituration  and 
abrasion.  The  beaches  now  forming  on  the  south-eastern 
coasts  of  England  are  in  great  part  composed  of  pebbles 
which  have  resulted  from  the  wear  and  tear  of  flints  derived 
from  the  chalk-cliffs.  The  puddingstone  of  Hertfordshire 
consists  of  flint  pebbles,  held  together  by  a  siliceous  cement. 
The  nagelfluhe,  formed  on  the  northern  flanks  of  the  Alps, 
consists,  in  great  part,  of  limestone  fragments,  partly  mixed 
with  fragments  of  quartz,  granite,  gneiss,  &c.  The  new-red 
conglomerate  of  the  Keuper  consists  mainly  of  pebbles  and 


3O2  Descriptive  Petrology. 

boulders  of  carboniferous  limestone,  usually  cemented  by 
dolomitic  matter,  whence  it  is  also  called  dolomitic  conglo- 
merate. The  pebble  beds  of  the  Bunter  are  formed  to  a 
large  extent  of  pebbles  of  quartz  and  quartzite,  in  a  matrix 
of  sandstone.  Conglomerates  occur  at  the  base  of  the  old- 
red-sandstone  in  Caithness  and  elsewhere.  These  contain 
not  merely  pebbles,  but  large  boulders,  and  the  deposit  is 
considered  by  Prof.  Ramsay  to  be  of  glacial  origin.  Con- 
glomerates are  sometimes  used  as  building-stones.  The 
dolomitic  conglomerate  of  Triassic  age  which  occurs  in  the 
Mendips  consists  of  pebbles  and  small  boulders  derived 
from  old-red-sandstone  and  carboniferous  rocks  and  the 
cementing  material  is  often,  but  not  invariably,  dolomitic. 
Conglomerates  have  a  special  geological  interest,  inasmuch  as 
they  usually  represent  old  sea-beaches,  and  consequently  indi- 
cate the  former  existence  of  coast  lines.  There,  are,  however, 
instances  in  which  they  may  not  represent  littoral  deposits. 

CALCAREOUS  TUFAS. 

Calcareous  tufa,  travertine,  pisolite,  osteocolla,  &c.,  are 
deposits  formed  by  the  chemical  precipitation  of  carbonate 
of  lime  from  waters  holding  bicarbonate  of  lime  in  solu- 
tion. Deposits  of  this  kind  are  generally  formed  in  the 
valleys  of  limestone  districts.  Any  foreign  bodies  which 
occur  in  the  solution  from  which  the  precipitation  takes 
place,  become  externally  incrusted,  just  as  kettles  and 
boilers  become  furred  internally  with  carbonate  of  lime. 
Successive  deposits  are  thus  formed,  and  the  result  is  a  light 
and  often  spongy  rock,  in  which  more  or  less  distinct  layers 
represent  the  successive  deposits.  Twigs,  leaves,  and  other 
objects  become,  in  this  manner,  incrusted  with  carbonate  of 
lime  ;  and  a  small  trade  is  carried  on  at  Matlock,  Knares- 
borough,  and  elsewhere,  by  submitting  natural  and  artificial 
objects  to  the  incrusting  influence  of  the  waters  of  these 
petrifying-wells,  as  they  are  termed.  The  variety  of  tufa 
named  osteocolla  consists  of  calcareous  deposits  around 


Tufas.     Siliceous  Sinters.  303 

twigs  and  mosses,  while  pisolite  is  composed  of  little  pea- 
like  spherical  concretions  of  carbonate  of  lime  around  small 
nuclei.  The  oolitic  limestones  have  also,  doubtless,  been 
formed  in  a  similar  manner,  although  under  very  different 
circumstances;  the  latter  representing  marine  deposits,  while 
the  ordinary  calcareous  tufas  are  usually  formed  in  valleys, 
and  constitute,  as  a  rule,  deposits  of  very  limited  extent.  In 
Italy,  however,  some  extensive  deposits  of  travertine  occur, 
especially  at  Tivoli,  where  the  waters  of  the  Anio  have  formed 
beds  of  tufa  four  or  five  hundred  feet  in  thickness.1  At 
this  place  spheroidal  concretions  from  six  to  eight  feet  in 
diameter  were  observed  by  Sir  Charles  Lyell. 

Calcareous  tufa  is  sometimes  used  as  a  building-stone, 
and  appears  to  be  very  durable,  even  in  old  edifices  built  by 
the  Romans. 

Siliceous  Sinters. 

These  are  rocks  formed  by  the  deposition  of  silica  from 
the  waters  of  hot  springs  and  geysers.  Geyserite  is  a  snow- 
white  siliceous  sinter  of  this  kind  which  occurs  incrusting 
the  pipes  of  geysers  and  forming  tolerably  thick  deposits  on 
the  adjacent  ground  over  which  the  water  of  the  geysers  is 
ejected.  The  deposits  of  siliceous  sinter  at  Rotomahana, 
near  Lake  Taupo,  in  New  Zealand,  are  perhaps  the  most 
wonderful  in  the  world.  At  this  place  the  thermal  waters 
charged  with  silica  in  solution  flow  down  the  hill-sides, 
forming  snow-white  terraces  of  siliceous  sinter.  The  in- 
fluence which  thermal  waters,  holding  silica  in  solution, 
have  exerted  upon  many  of  our  older  rocks,  is  a  question 
which  well  deserves  the  attention  of  petrologists. 

MINERAL  DEPOSITS  CONSTITUTING  ROCK-MASSES. 

Rock-salt,  in  some  districts,  constitutes  deposits  of  great 
thickness  :  coal  also  forms  beds  or  seams  of  variable  thick- 
ness in  the  carboniferous  series,  and  in  rocks  of  oolitic  and 
miocene  age  in  certain  countries. 

1  Lyell's  Principle*  of  Geology,  pth  edition,  p.  244. 


304  Descriptive  Petrology. 

Gypsum  sometimes  forms  beds  of  considerable  thickness. 
Iron  ores  occasionally  occur  in  large  masses,  as  in  the  case 
of  the  Pilot  Knob,  Missouri,  which  consists  almost  wholly 
of  hematite,  of  some  of  the  large  deposits  of  this  ore  at 
Norberg  and  Langbanshytta  in  Sweden,  Gellivara  in  Lap- 
land, and  Santander  in  Spain,  and  of  the  extensive  deposits 
of  limonite  and  clay-ironstone  which  occur  in  various  parts 
of  the  world.  Felspars,  such  as  labradorite,  sometimes  con- 
stitute, by  themselves,  rock-masses  of  considerable  thickness 
and  extent.  Mr.  Bauerman  states  that,  in  Labrador,  rocks 
consisting  sometimes  of  labradorite,  at  others  of  oligoclase, 
form  large  and  important  beds.  Various  metallic  ores  at 
times  constitute  lodes  and  beds  of  considerable  magnitude, 
but  for  a  description  of  them  the  student  must  refer  to 
larger  works,  treating  more  fully  upon  this  part  of  the 
subject,  which  rather  belongs  to  mineralogy  and  mining  than 
to  petrology.  > 

The  deposits  of  cinnabar  in  Spain,  of  zinc  ores  in  New 
Jersey,  U.S.,  and  in  the  Harz,  of  copper  in  the  Lake 
Superior  district,  and  of  other  minerals  of  the  heavy  metals 
which  occur  more  or  less  plentifully  in  all  parts  of  the  world, 
the  deposits  of  rock-salt  in  Russia,  Poland,  Gallicia,  Ger- 
many, Austria,  Spain,  England,  Algeria,  Abyssinia,  and  in 
various  parts  of  America,  the  phosphatic  beds  met  with  in 
certain  formations,  and  the  important  occurrences  of  coal, 
lignite,  bituminous  schists,  asphaltum,  and  petroleum,  are 
matters  which  can  be  studied  in  most  geological,  minera- 
logical,  and  mining  books.  Ice,  which  must  be  regarded  as  a 
rock,  occurs  in  thick  and  extensive  sheets  over  the  Arctic  and 
Antarctic  regions,  while  perennial  snow  exists  at  great  ele- 
vations, even  at  the  equator.  Ice  is  sometimes  interstratined 
with  sands,  &c.,  as  in  some  parts  of  Siberia.  Deception 
Island,  in  New  South  Shetland,  lat.  62°  55'  S.,  is  principally 
composed  of  alternate  layers  of  volcanic  ashes  and  ice,  and 
similar  alternations  of  beds  have  been  observed  on  Cotopaxi, 
while  a  large  glacier  has  been  discovered  by  Gemmellaro, 


Ice.  305 

beneath  a  lava- stream,  at  the  foot  of  the  highest  cone  of 
Etna.  The  latter  phenomenon  is  described  in  Lyell's  *  Prin- 
ciples of  Geology,'  and  the  author  attributes  the  preservation 
of  the  glacier  to  a  mass  of  snow  having  been  covered  by  a 
badly  conducting  layer  of  volcanic  ash,  scoriae,  &c.,  prior  to 
the  eruption  of  the  lava  which  now  caps  the  glacier.  Ice 
exhibits  stratification  in  the  upper  portions  of  glaciers,  but 
the  motion  of  the  latter  in  their  descent  to  lower  levels 
gradually  obliterates  these  traces  of  bedding.  Glaciers  and 
ice-sheets,  in  creeping  over  the  subjacent  rocks,  scrape  them 
into  smooth,  hummocky  forms,  termed  roches  moutonnees,  by 
means  of  the  stones,  gravel,  &c.,  which  get  imprisoned  be- 
tween the  ice  and  the  rock-surface  over  which  it  moves. 
The  phenomena  of  glaciation  will  be  found  fully  discussed 
in  many  books  and  papers  which  have  been  published  on 
the  subject  during  the  last  ten  or  fifteen  years. 


APPENDIX. 


A. 

SINCE  this  book  was  written,  Mr.  Watson,  of  Pall  Mall,  has,  at 
my  suggestion,  constructed  a  microscope  specially  suited  for 
petrological  work. 

This  instrument  is  in  one  respect  decidedly  preferable  to  the 
microscopes  commonly  made  on  the  Continent,  inasmuch  as  it  is 
supported  upon  trunnions,  like  the  ordinary  English  microscopes, 
and  consequently  allows  of  any  inclination  of  the  working  part  of 
the  instrument.     The  foot,  up  to  the  trunnion-bearings,  is  cast 
in  one  piece,  after  the  model  of  Ross,  Crouch,  and  other  well- 
known  makers.     Upon  this  a  brass  limb  is  supported.     The 
limb,  below  the  trunnions,  is  cylindrical,  and  carries  an  ordi- 
nary mirror  with  a  jointed  arm.     The  limb,  above   the  axis, 
describes  such  a  curve  as  is  most  convenient  for  lifting  and 
carrying  the  instrument,  without  incurring  any  risk  of  strain  to 
the  working  parts.     The  upper  portion  of  the  limb  is  planed 
to  receive  the  rackwork,  which  constitutes  the  coarse  adjust- 
ment, in  a  manner  similar  to  that  employed  in  the  construc- 
tion of  the  Jackson-pattern  microscopes,  as  in  the  instruments 
by  Beck.     The  tube  or  body  carries  the  rack,  and,  by  it,  is 
moved  against  these   planed  surfaces,  for  focussing.     At  the 
lower  end  of  the  tube,  immediately  above  the  thread  which 
carries  the  objective,  a  slot  is  cut  to  receive  a  Klein's  quartz 
plate.     The  quartz  plate  is  in  a  small  brass  mount,  which  fits 
this  slot,  and  can  be  removed  from  the  instrument  at  pleasure. 
At  such  times  a  revolving  collar  can  be  turned  over  the  aperture. 
The  eye-piece,  at  the  upper  end  of  the  tube,  is  made  with  a 
disc,  about  i|  inch  diameter,  having  its  outer  edge  divided,  and, 
immediately  above  this,  a  similar  disc,  connected  with  the  eye- 

X  2 


308  Appendix. 

piece  analyser,  revolves  with  an  index,  so  that  the  analyser  can 
be  set  in  any  desired  position,  or  the  amount  of  rotation 
imparted  to  it  can  be  recorded.  The  eye-piece  is  also  furnished 
with  crossed  cobwebs  for  centering.  A  space  is  left  between 
the  bottom  of  the  analyser  and  the  eye-glass,  sufficient  to  permit 
the  introduction  of  a  plate  of  calcspar  for  stauroscopic  examina- 
tions, and  an  eye-piece-fitting,  without  lenses,  is  also  supplied, 
so  that  the  instrument  can,  by  the  superposition  of  convergent 
lenses  over  the  polariser,  be  used  for  viewing  the  systems  of 
rings  and  interference  crosses,  presented  by  crystals  when 
examined  by  convergent,  polarised  light.  The  polarising  prism 
is  mounted  upon  a  movable  arm,  beneath  the  stage,  and  carries 
a  graduated  disc,  so  that  it  can  be  set  in  any  desired  direction, 
or  be  instantly  displaced  when  ordinary  illumination  is  requisite. 
The  stage  of  the  microscope  is  circular,  and  can  be  rotated  and 
centered.  It  is  divided  on  silver  to  half  degrees,  and  a  vernier  is 
attached  to  the  front  of  the  stage  for  goniometric  purposes.  It 
has  also  two  rectangular,  divided  lines,  to  serve  as  a  finder. 
The  centring  is  effected  by  two  screws  working  against  a  spring 
on  the  opposite  side.  These  screws  enable  the  observer  to 
centre  the  instrument  for  any  objective.  They  are  conveniently 
situated,  so  as  not  to  be  liable  to  derangement  during  the 
ordinary  manipulation  of  the  instrument.  The  fine  adjustment 
carries  a  divided  head,  for  the  approximate  measurement  of 
the  thickness  of  sections.  The  body  of  the  microscope  can 
be  clamped  in  any  position  by  a  lever,  attached  to  the  right 
trunnion. 

In  the  instrument  which  I  have  examined  the  adjustments 
work  very  smoothly.  The  thread  for  the  objectives  is  of  the, 
now,  almost  universal  gauge,  so  that  any  English  objectives  may 
be  used.  Foreign  ones  can  also  be  employed,  by  means  of  an 
adapter. 

The  engraving  on  page  306,  for  which  I  am  indebted  to  Mr. 
Watson,  shows  the  general  plan  upon  which  the  instrument  is 
constructed.  The  smaller  figures  represent  the  quartz  plate, 
the  calcspar  plate,  and  a  section  of  the  polariser  and  its  fittings. 


Appendix.  309 

B 

Books  consulted  in  the  preparation  of  this  Work. 

Ansted,  D.  T.   '  Elementary  Course  of  Geology  and  Mineralogy.' 
London,  1850. 

Ansted,  D.  T.     '  Applications  of  Geology  to  the  Arts  and  Manu- 
factures.'    London,  1865. 

Blum,  J.  R.     « Handbuch   der   Lithologie  oder   Gesteinslehre.' 
Erlangen,  1860. 

Boficky,  E.     '  Petrographische  Studien  an  den  Basaltgesteinen 
Bohmens.'     Prag,  1874. 

Boficky,  E.     '  Petrographische   Studien  an   den   Phonolithge- 
steinen  Bohmens.'     Prag,  1873. 

Boficky,  E.     '  Petrographische    Studien   an   den  Melaphyrge- 
steinen  Bohmens.'     Prag,  1876. 

Boficky,  E.  c  Elemente  einer  neuen  chemisch-mikroskopischen 
mineral-  und  Gesteins-Analyse.'     Prag,  1877. 

Brewster,  D.    l  Optics'  ('  Cabinet  Cyclopaedia').  London,  1831. 
Bristow,  H.  W.     *  Glossary  of  Mineralogy.'     London,  1 86 1. 

Bryce,  J.     '  Geology  of  Arran  and  other  Clyde  Islands.'     Glas- 
gow and  London,  1872. 

Coquand,  H.     '  Traitd  des  Roches.'     Paris,  1857. 

Cotta,  B.  von.     '  Rocks  Classified  and  Described.'   Translation 
by  P.  H.  Lawrence.     London,  1866. 

Credner,  H.     'Elemente  der  Geologic'     Leipzig,  1876. 

Dana,  J.  D.     '  System  of  Mineralogy.'    Fifth  Edition.     London 

and  New  York,  1871. 
De  la  Beche,  H.     '  How  to  Observe — Geology.'    London,  1836. 

De  la  Beche,  H.     '  Report  on  the  Geology  of  Cornwall,  Devon, 
and  West  Somerset.'     London,  1839. 

De  la  Beche,  H.  '  Researches  in  Theoretical  Geology.'  London, 

1834. 
De  la  Fosse.   '  Nouveau  Cours  de  Mineralogie.'  Paris,  1858-62. 


3io  Appendix. 

Delesse,  A.    '  Recherches  sur  1'Origin  des  Roches.'  Paris,  1865. 

Delesse,  A.  l  Recherches  sur  les  Pseudomorphoses.'  l  Annales 
des  Mines/  XVI.  Paris,  1859. 

Delesse,  A.  and  De  Lapparent.     '  ReVue  de  Gdologie.'     Paris. 
Descloizeaux,  A.     '  Manuel  de  Mineralogie.'     Paris,  1862. 

Fischer,  H,  '  Kritische  mikroskopisch-mineralogische  Studien.' 
Freiburg,  1869  and  1871. 

Ganot.  '  Elementary  Treatise  on  Physics.'  Translation  by 
Atkinson.  London,  1863. 

Green,  A.  H.     '  Geology  for  Students.'     Parti.    London,  1876. 

Hunt,  R.  '  Mineral  Statistics  of  the  United  Kingdom,  1858. 
Part  II.  Embracing  Clays,  Bricks,  &c.,  Building  and  other 
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Jannetaz,  E.     '  Les  Roches.'    Paris,  1874. 

Judd,  J.  W.  '  Geology  of  Rutland,  &c.'  (Mem.  Geol.  Surv.) 
London,  1875. 

Jukes,  J.  B.     '  Student's  Manual  of  Geology.'   Edinburgh,  1862. 
Kengott,  A.     '  Elemente  der  Petrographie.'     Leipzig,  1868. 
Kinahan,  G.  H.    '  Handy-Book  of  Rock  Names.'  London,  1873. 

La  Vallde  Poussin  and  Regard,  A.  '  Mdmoire  sur  les  Roches 
dites  Plutoniennes  de  la  Belgique  et  de  1'Ardenne  Franchise.' 
(Acad.  Royale  des  Sciences  de  Belgique.)  Bruxelles,  1876. 

Lasaulx,  A.  von.     '  Elemente  der  Petrographie.'    Bonn,  1875. 

Lommel,  E.  ( The  Nature  of  Light,  with  a  General  Account  of 
Physical  Optics.'  London,  1875. 

Lyell,  C.     '  Principles  of  Geology.'  9th  Edition.    London,  1853. 
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Pereira,  J.  P.     'Lectures  on  Polarised  Light.'     London,  1843. 
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Appendix.  31 1 

Pinkerton,  J.     <  Petralogy.'     London,  1811. 

Playfair,  J.  '  Illustrations  of  the  Huttonian  Theory  of  the 
Earth/  Edinburgh,  1802. 

Ramsay,  A.  C.  '  Physical  Geology  and  Geography  of  Great 
Britain/  3rd  Edition.  London,  1872. 

Renard,  A.  '  Memoire  sur  la  Structure  et  la  Composition 
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Belgique.)  Bruxelles,  1877. 

Rosenbusch,  H.  '  Die  Steiger  Schiefer  und  ihre  Contactzone 
an  den  Granititen  von  Barr-Andlau  und  Hohwald.'  Strass- 
burg,  1877. 

Rosenbusch,  H.  '  Mikroskopische  Physiographic  der  petro- 
graphisch  wichtigen  Mineralien.'  Stuttgart,  1873. 

Rosenbusch,  H.     '  Mikroskopische  Physiographic  der  massigen 

Gesteine.'     1877. 

Roth,  J.     '  Die  Gesteins-Analysen.'    Berlin,  1861. 
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Smith,  C.  H.  *  Lithology,  or  Observations  on  Stone  used  for 
Building.'  London,  1845. 

Spottiswoode,  W.  '  Polarisation  of  Light '  (Nature  Series). 
London,  1874. 

Tyndall,  J.  '  Six  Lectures  on  Light.'  Delivered  in  America. 
London,  1875. 

Tyndall,  J.  '  Notes  on  a  Course  of  Nine  Lectures  on  Light.' 
Delivered  at  the  Royal  Institution.  London,  1872. 

Vogelsang,  H.     l  Die  Krystalliten.'     Bonn,  1875. 

Ward,  J.  C.  « Geology  of  the  Northern  Part  of  the  English 
Lake  District.'  (Mem.  Geol.  Surv.)  London,  1876. 

Whitaker,  W.     '  Geological  Record.'     London. 

Woodward,  C.    '  Familiar  Introduction  to  the  Study  of  Polarised 

Light.'     London,  1861. 
Woodward,  H.  B.    '  Geology  of  England  and  Wales.'    London, 

1876. 
Woodward,  H.  B.     '  Geology  of  East  Somerset  and  the  Bristol 

Coal-Fields.'     (Mem.  Geol.  Surv.)     London,  1876. 


312 


Appendix. 


Zirkel,  F.     '  Lehrbuch  der  Petrographie.'     Bonn,  1866. 

Zirkel,  F.     '  Microscopic  Petrography.'     (U.  S.  Geol.  Explora- 
tion of  Fortieth  Parallel.)     Washington,  1876. 

Zirkel,  F.     '  Mikroskopische  Beschaffenheit  der  Mineralien  und 
Gesteine.'     Leipzig,  1873. 

Zirkel,  F.     '  Untersuchungen   iiber  mikroskopisch  Zusammen- 
setzung  und  Structur  der  Basaltgesteine.'     Bonn,  1870. 

Also  numerous  papers  published  in  the '  Quarterly  Journal  of 
the  Geological  Society/  the  '  Geological  Magazine,'  '  Leonhard's 
Jahrbuch  fiir  Mineralogie/  'Zeitschrift  der  deutschen  geolo- 
gischen  Gesellschaft,'  'Annales  des  Mines,'  and  numerous  other 
publications. 

Among  these,  books  and  papers  by  the  following  authors 
have  been  specially  consulted : 

Allport,  S. 
Bonney,  T.  G. 
Boficky,  E. 
Cotta,  B.  v. 
Dana,  J.  D. 


De  la  Beche,  Sir  H. 
Delesse,  A. 
Forbes,  D. 
Judd,  J.  W. 


Kengott,  A. 
Lasaulx,  A.  v. 
Lyell,  Sir  C. 
Phillips,  J.  A. 
Rdnard,  A. 
Rosenbusch,  H. 
Sorby,  H.  C. 
Zirkel,  F. 


Errata. 

Page  8,  line  20,  for  marine  read  submarine. 
»  I03»    »    33>    ,,  regarded  „    sometimes  regarded. 
„  220,     „    33,    ,,  basalt       „    mica-basalt. 


INDEX. 


ACI                                              BAT                                              BYS 

ACID,    sub    class    of 
eruptive  rocks,  34, 

Arenaceous  limestone,  20 
—  sedimentary  rocks,  276 

Bath  stone,  286 
Baveno  type  of  twinning, 

177 

Argillaceous      limestone, 

92 

Actinolite,  131 

20 

Beale,  Lionel  S.,  59 

—  schist,  295 

—  rocks,  282 

Beale's  reflector,  49 

Aden,   globular  silica  in 

Arkose,  280 

Belonites    in    pitchstone, 

quartz-trachyte  of,    152 
Age  of  strata,  determined 

Arran,  pitchstone  of,  196 
Arranging     rock     speci- 

190 
Berkum,      near       Bonn, 

by  means  of  fossils,  30 

mens,  39 

sanidine-rhyolite        of, 

Aislaby  sandstone,  280 

Asbestus,  131 

224 

Albite,  97 

Ascension,    obsidian    of, 

Binstead  limestone,  289 

—  type  of  twinning,  98 

181 

Biotite,   135 

Alderley  Edge,  sandstone 

Ashes,  volcanic,  267 

Blowpipe  apparatus,  45 

of,  279 

Asparagus  stone,  147 

Bobenhausen,     tachylyte 

Allport,  S.,  107,  122,   152, 

Aubuisson,  209 

of,  201 

182,   190,   193,  197,  245, 

Augen-gneiss,  295 

Bombs,  volcanic,  267 

260 

Augite-andesite,  236 

Bonney,  T.  G.,  182,  193, 

Altered  conditions  of  py- 

— porphyrite,  240 

211,  260,  262,  270 

roxene,  126 

—  syenite,  218 

Boficky,    £.,    method   of 

—  eruptive  rocks,  269 

Aussig,    tridymite    from, 

analysis,  100 

—     sedimentary      series, 

152 

120,   125,   132,   229, 

289 

Auvergne,  domite  of,  226 

253,  260 

Amazon  stone,  97 

Axes  of  elasticity,  78 

Bouteillenstein,  187 

Amianthus,  131 

—  optical,  78 

Box  Hill  stone,  286 

Amorphous  matter,  170 

Axiolitic  structure  in  vit- 

Bradford flags,  278 

Amphibole,  127 
Amphibolite,  298 

reous  rocks,  184 

.Bramley  Fall  stone,  278 
Brazen     Tor,     granulitic 

Amphigene,  108 

rock  of,  296 

Analcime,  159 

BAGSHOT  sands,  281 

Breccias,  299 

Anamesite,  252 

Banded  structure  in 

—  defined,  17 

Andalusite,  143 

vitreous  rocks,  181 

Breislackite,  125 

Andesine,  99 
Andesite,  234 

Bangor  quarries,  283 
Bannisdale  slates,  283 

Breithaupt,  97 
Brent     Tor,       schalstein 

Anisotropic  substances,  77 

Bargate  stone,  280 

near,  248 

Anorthite,  98 

Basalt,  252 

volcanic      ejecta- 

Ansted,  277 

—  classification  of,  253 

menta  of,  268 

Anticlinal,  term   defined, 

—  columnar  structure  of, 

Brewster,  142,  150,  159 

21 

Antrim,  chalk  of,  290 

258 
—  diagram  of  deviations 

Brezina,    stauroscope  of, 
83 

—  keuper   sandstone   of, 

from,  261 

Bristol  Coal-Field,   sand- 

280 

Basaltite,  252 

stones  of,  278 

Apatite,  145 

Basanite,  293 

Buch,  L.  von,  171,  234 

Aplite,  211 

Basic  rocks,  34 

Bunsen,  250 

Aqueous    rocks    defined, 

—  sub-class  of  rocks,  177 

Bunter  sandstones,  279 

15 

Basis,  term  defined,  168 

Buschbad,  perlite  of.  183 

Arbroath  sandstone,  277 

Bath  bricks,  281 

Byssolite,  131 

Index. 


CAB                                               DAR                                              ERL 

CABINETS,  43 
Caithness  flagstone, 

Cliffs    and    escarpments, 
27 

Dathe,  J.  F.  E.,  87,  245 
Daubeny,  37 

277 

Clinkstone,  228 

Definition    of    the    term 

Calabria,  mica  syenite  of, 

Clinochlore,  137 

rock,  6 

218 

Clip-lens,  44 

De  la  Beche,  Sir  H.,37 

Calc-aphanite,  247 
Calcareous  rocks,  285 

Coal,  preparation  of  sec- 
tion of,  71 

De  la  Fosse,  78 
Delesse,  A.,  130,  171,  221, 

—  sandstone,  19 

Coast    lines  affected    by 

269 

—  tufa,  1  8,  302 

relative     hardness      of 

Dent  marble,  285 

Calcite,  148 

rocks,  27 

Denudation,  25 

Calcspar,  148 
—  fluid  inclosures  in,  149 
—  twinning  of,  149 
Cambrian  sandstones,  276 

changes  in,  u 
Collecting     rock      speci- 
mens, 39 
Columnar     structure      of 

Descloizeaux,  96,  97,  120, 

122,   135,   ISO,   159,    l6o 

Devitrification,  185 
—  of  pitchstones,  198 

—  slates,  283 

basalt,  258 

Devonian  limestones,  285 

Canada,   gneissic  syenite 
of,  218 

—  in  phonolite,  233 
Collyweston  slates,  283 

—  sandstones,  277 
—  slates,  284 

—  moroxite  of,  147 
Camera  lucida,  49 

Condensers,  49 
—  achromatic,  50 

Devonshire,  schalstein  of, 
248 

Canary  islands,  eutaxite 

Conglomerate,     17,      19, 

Diabase,  244 

of,  232 

297 

—  amygdaloidal,  247 

Cancrinite,  108 

—  dolomitic,  302 

—  aphanite,  247 

Cape  Wrath,  amphibolite 
schist  of,  218 
Carbonic  acid,  action  of, 

—  phonolite,  233 
Coniston  flags,  283 
Contortion  of  strata,  21 

—  porphyrite,  239 
—  schist,  247 
Diabasmandelstein,  247 

upon  certain  rocks,  29 
Carboniferous   limestone, 

Cookworthy,  Wm.,  273 
Copper  pyrites,  157 

Diallage,  124 
—  andesite,  234 

285 

Corals,  1  8 

Diallagoid  augite,  132 

—  sandstone,  278 

Cordier,  209 

Dimetian  basalts,  262 

—  slates  and  flags,  284 
Carlow  flags,  278 

Cordierite  granite,  210 
Cornubianite,  213 

Dinorwig  quarries,  283 
Diorite,  241 

Carlsbad    type    of   twin- 

Corriegills, Arran,  pitch- 

—  porphyrite,  238 

ning,  92 

stone  of,  197 

Dip,  term  defined,  21 

Cassiterite,  148 

Corsham  Down  stone,  286 

Disthene  rock,  263 

Craigleith  sandstone,  278 

Coticule,  142 

Disturbances       of       the 

Cast-iron  plate  for  grind- 
ing sections,  61 

Cotopaxi,  304 
Cotta,  B.  von,  219,  220 

earth's  crust,  9 
_D«lei»ite,  252 

Centering  57 

Credner,  H.,  206,  288 

Dolomites,  285 

Chabasite,  160 

Cretaceous      sandstones. 

Dolomitic   conglomerate, 

Chalcopyrite,  157 

280 

302 

Chalk,  289 

Cross-hatching  in   ortho- 

Domite,  226 

Chalk  of  Antrim,  290 

clase,  94 

Doulting  stone,  286 

Chert,  285 

Crust  of  the  earth,  dislo- 

Drachenfels,       tridymite 

Chiastolite,  143 

cations  of,  ii 

from,  152 

—  slate,  291 

Crypto-crystalline  matter, 

Dressel,  114 

China  clay,  272,  284 

170 

Dundee  sandstone,  277 

formation  of,  30 

Crystalline  eruptive  rocks, 

Dunite,  265 

Chips  for  section-cutting, 

202 

Durocher's  theory,  34 

60 

—  limestones,  292 

Chlorite,  136 

Crystallites,  160 

—  in  quartz,  151 
—  schist,  297 

—  in  obsidian,  187 
Cubic   system,   cleavages 

EARTHQUAKES,  n 
Eifel,   leucitophyrs 

Chrysolite,  116 

in,  171 

of,  2  6 

—  pseudo,  187 

Czertochin,         Bohemia, 

Eisenglimmer,  155 

Classification    of    rocks, 

tachylyte  of,  202 

Eklogite,  263 

174 

Elba,   specular    iron    of, 

Clastic  rocks,  299 

!55 

Clays,  17,  282 

DACITE,    quartzose, 

Elaeolite,  108 

—  of  Bovey  Beds,  284 

234 

Elevation  and  subsidence 

—  &c.,  washing  of,  73 

Damascened  structure  in 

of  land,  ii 

Cleavage,  cause  of,  35 

vitreous  rocks,  181 

El  van,  204,  210 

—  in  rocks,  12 

Damourite,  134 

Encrinital  marble,  285 

Cleavages,    diagrams    of, 

Dana,    J.    D.,    90,     117, 

Enstatite,  120 

—  of  minerals,  171 

121,  131,  147,  257 
Darwin,  C,  36,  37,  212 

Epidote,  127,  139 
Erlan,  296 

Index. 


315 


ERU                                               HAP                                               INT 

Eruptive    rocks,   general 
characters  of,  32 

Friction  breccias,  300 
Fritsch  and  Reiss,  233 

Hardness,  scale  of,  44 
Hart  Hill,  sandstone   of, 

Eruptive  rocks,  vitreous, 

Fntzgartner,    M.  G.   R., 

278 

177 

117,  146 

Hartley,  W.  N.,  164 

Escarpments    and    cliffs, 

Fruchtschiefer,  292 

Haselgebirge,  300 

27 

Fuchsite,  134 

Haughton,  S.,   113,  207, 

Estimation  of  amount  of 

Fuller's  earth,  284 

208,  257 

waste    by   denudation, 

Fusion  of  vitreous  matter, 

Hauyne,  112 

24 

179 

—  and  nosean,  232 

Etna,  ashes  of,  267 

—  basalt,  257 

—  glacier  on,  304 

Hauynophyr,  257 

—  lavas  of,  255,  267 

/~*  ABBRO,  249 

Hawaii,  filiform  lava   of, 

Eulysite,  263 

vJT     Garbenschiefer,  292 

186 

Eurite,  209,  214 

Garnet,  140 

—  obsidian  of,  191 

Eutaxite,  233 

—  rock.  262 

Headon  Hill  sands,  281 

Eye-pieces,  52 

Gemmellaro,  304 

Hematite,  155,  304 

Geyserite,   303 

Henslow,  212 

Geysers,  33,  303 

Hertfordshire,    pudding- 

FALSE  bedding,  15 
Faroe     Isles,    heu- 

Giant's  Causeway,  basalt 
of,  258 

stone  of,  301 
Heulandite,  160 

landite  of,  160 

Glass  inclosures,  165 

Hexagonal  system,  cleav- 

Faults, origin  of,  13 
Felsi-dolentes,  34 

—  natural,  170 
Globular  silica,  152 

ages  in,  172 
Hills      and     mountains, 

P'elsitic  matter,  167 

Globulites,  161 

causes    affecting    their 

—  pitchstone,  197 
Felsite,  168,  213 

Gneiss,  212,  294 
—  augen,  295 

forms,  29 
—  and  valleys,  formation 

Felspars,  86  _    ' 

—  foliation  in,  212 

of,  28 

—  decomposition  of,  30 

—  protogine,  212,  295 

Himalayas,  sandstone  of, 

Felspar-porphyries,  209 
—  rocks,  304 

—  syenitic,  295 
Gneissic  syenite,  218 

281 
Homogeneous      vitreous 

Felstune,  209,  214 

Goniometers    for    micro- 

rocks, 1  80 

Ferrite,  167 

scopes,  53 

Horizontal    strata,     map 

Ferruginous      sandstone, 

Granite,  202 

and  section  of  district 

19 
Filiform  condition  of  vit- 

— diagram  of  deviations 
from,  215 

composed  of,  22 
denudation  of,  23 

reous  lavas,  1  86 

—  origin  of,  206 

Hornblende,  128 

Finders,  51 

Granitell,  211 

—  andesite,  234 

Fingal's  Cave,  258 

Granitic  type,  deviations 

—  schist,  295,  298 

Fire-stone,  281 

from,  213 

—  syenite,  217 

Flags,  282 

Granitite,  210 

Hornblendic  granite,  203 

—  Silurian,  283 

Granular  diabase,  247 

Hydrometamorphism,  208 

Fleckschiefer,  292 

Granulite,  211,  295 

Hypersthene,  119 

Flexure  of  strata,  21 

Green,  A.  H.,  37 

—  andesite,  234 

Flints,  281 

Greenstone,  241 

Hypersthenite,  250 

Fluid  inclosures,  164 

—  tuffs,  249 

Hull,  E.,  257,  277 

Fluxion  structure,  163 

Greisen,  211 

Hungary,    tridymite     of, 

Foliated  rocks,  293 

Grinding  of  rock  sections, 

152 

Foliation,  36 

65 

Hunt,  R.,  280 

Foraminifera,  18 

Grit,  17,  19 

—  T.  Sterry,  36,  271 

Forbes,  David,  2 

Groth,  140,  150 

•    theory    of   fels- 

Forces operating   in    the 

Groundmass,     term     de- 

pars, 95 

interior  of  the  earth,  10 

fined,  1  68 

Formation,  term  defined, 

Gas  inclosures,  164 

30 

Gumbel,  155,  248 

TCE,  304 

Fossiliferous    rocks     de- 

Gypsum, 304 

_L     Idocrase,  142 

fined,  15 

Inclined  strata,   denuda- 

Fossils indicative  of  con- 

tion of,  23 

ditions     under     which 
rocks  have  been  depo- 

HALB-GRANIT, 211 
Halleflinta,      209, 

Inclosures  in  quartz,  150 
—  of  fluid,  164 

sited,  15 

214 

—  of  glass,  165 

Fossils  replaced  by  py- 

Halifax flags,  278 

Indicator  for  eye-piece,  52 

rites,  157 

Hammers,  40 

Internal  heat  of  the  earth, 

Foster,  C.  Le  Neve,  212 

Hassock,  281 

evidence  of,  10 

Fragmental  rocks,  299 

Hastings  sands,  280 

Interpositions  in  felspars, 

Freiberg,  gneiss  of,  294 

Haplite,  211 

95 

Index. 


INT                                              MAR 

Intrusive     sheets    distin- 

Laminar fission,  12 

guished  from  lava-flows, 

Lapilli,  267 

32 

Lasaulx,  A.  von,  59,  103, 

Iron  glance,  155 

114,  119,  133,  195,  216, 

—  ores,  304 

218,  231,  232,  234,  247, 

—  pyrites,  156 

255,  264,  290 

Isle    of   Wight,    tertiary 

Latente,  272 

limestone  of,  289 
Isotropic  substances,  76 

Lava-flows  distinguished 
from    intrusive    sheets, 

Itacolumite,  297 

32 

Italian  marbles,  287,  293 

Lavas  of  Etna,  255 

Vesuvius,  256 

Lebour,  G.  A.,  32 

TADE,  131 

Leeson's  microscope,  59 

J      Jasper,        porcelain, 
290 

Lepidolite,  134 
Lepidomelane,  136 

Jervis,  W.  P.,  293 

Leptinite,  211,  295 

Jointing,  12 
Jordan's    section    cutter, 

Leucilite,  256 
Leucite,  108 

61 

—  basalt,  256 

Judd,  J.  W.,  37,  191,  193, 

Leucitophyr,  256 

262,  280 

Leucoxene,  155 

Jukes,  J.  B.,  13 
Jurassic  sandstones,  280 

Levigation,  73 
Levy,  Michel,  152 

Lherzolite,  263 

Liassic  clays,  284 

KALKOWSKY,  171 

—  limestones,  286 

Kaolin,    203,  272, 

Limestones,  17,  20 

284 

Limestone,  carboniferous, 

—  formation  of,  30 

285 

Kaolinite,  284 

—  cretaceous,  288 

Katzenbuckel,       nepheli- 

—  crystalline,  287,  293 

nite  of,  255 

—  Devonian,  285 

Kengott,  120,  219 
Kersantite,  220,  239 

—  magnesian,  20,  285 
—  oolitic,  286 

Kersanton,  220,  239 

—  Silurian,  285 

Ketton  stone,  286 

—  tertiary,  289 

Keuper  sandstone,  279 

Limonite,  156 

Kieselschiefer,  293 

Lincolnshire  oolites,  286 

Kilanea,  lavas  of,  199 

Lipari,  obsidian  of,   183, 

Kinahan,  G.  H.,  209,  238 

189 

Kinzigite,  262 

Liparite,  222 

Kjerulf,  257 

Lithia  mica,  134 

Klaproth,  228 

Lithoidite,  222 

Klein's  quartz-plate,  58 
Knaresborough,    calcare- 

Llanberis quarries,  283 
London  clay,  284 

ous  tufa  of,  302 
Knotenschiefer,  292 

Luxullianite,  210 
Lydian  stone,  293 

Kobell,    V.,    stauroscope 

Lyell,  SirC.,  IT,  37,  303, 

of,  81 

305 

Kupferberg,   bronzite  of, 

121 

Kyanite,  144 
—  in  quartz,  151 

MACHINES    for 
grinding       micro- 

scopic sections,  61 

Made,  143 

LAACHER      See, 
hauyne-basalt  0^257 

Magma,     term     defined, 
1  68 

hauyne  of,  114 

Magnesian  limestone,  20, 

Labelling  specimens,  42 

285 

Labradorite,  99 

Magnetite,  153 

Lake  District,  mica  traps 
of,  220 

Magnets,  45 
Maltwood's  finder,  51 

-     volcanic     ejecta- 

Marble,  encrinital,  285 

menta  of,  268 

Marbles,  Italian,  287,  293 

NEP 


Marbles,  microscopic  cha- 
racter of,  287 

Marcasite,  157 

Margarodite,  135 

Marialite,  112 

Marine  denudation, 
effects  of,  25 

Marl,  19 

Maskelyne,  N.  S.,  120 

Matlock,  calcareous  tufa 
of,  302 

Medway,  mud  of,  285 

Meionite,  112 

Melaphyre,  260 

Merrifield,  C.W.,  259 

Metamorphism,  36,  208 

Mica  basalt,  257 

Micaceous  felstone,  209 

Micas,  132 

Mica  schist,  297 

—  syenite,  218 
Microcline,  96 
Micro-crystalline  matter, 

—  felsitic  matter,  171 
Microliths,  162,  185 

• —  in  perlite,  194 
Microscopes,  46 
Microscopic    analysis    of 
E.  Boiicky,  100 

—  preparations,  59 
Millstone  lavas,  256 
Mineral  deposits,  303 
Minerals,  optical  charac- 
ters of,  74 

—  rock-forming,  86 
Minette,  219 
Miocene  sandstones,  281 
Missouri,     hematite     of, 

156,  304 

Mohl,  H.,  152,  252 

Monoclinic  system,  cleav- 
ages in,  172 

Moraines,  301 

Moroxite,  147 

Morris,  J.,  281 

Mountains  and  hills, 
causes  affecting  their 
forms,  29 

Mounting  sections  of 
rocks,  &c.,  69 

Mudstones,  17 

Murchison,  Sir  R.  I.,  212 

Muscovite,  133 

Museums,  arrangement  of 
rock  collections  in,  43 

NAPLES,  piperno  of, 
233 

Nassau,  schalstein  of,  248 
Natrolite,  159 
Negative  crystals,  165 
Nepheline,  104 

—  alteration  of,    into  na- 
trolite,  159 


Index. 


317 


NEP                                              PYR 

Nepheline  basalt,  255 

Perlitic  structure,  182 

Pyrometamorphism,  208 

Nephelinite,  255 
Neurode,     hypersthenite 

—  .  —  in  tachylyte,  194 
Permian  limestones,  285 

Pumice,  191,  267 
Purbeck  limestone,  286 

of,  251 

—  sandstones,  278 

—  marble,  286 

Nevada,    U.S.,   vitreous 
rocks  of,  184 

Perthite,  97 
Petrosilex,  209,  214 

Pyramids,        nummulitic 
limestone  of  the,  289 

New    red    conglomerate, 

Petworth  marble,  287 

Pyroxene,  121 

301 

Phillips,  J.,  36 

sandstone,  279 

—  J.  A.,  37,  70,  151,  211 

Newton,  E.  T.,  71 
Niedermendig,    lava    of, 

Phlogopite,  135 
Phonolite,  228 

QUARTZ,  149 
—  diabase,  247 

256 

—  classification  of,  229 

—  inclosures  in,  150 

Nile  mud,  282 

—  conglomerate,  233 

—  porphyry,  210 

Norite,  251 

—  tuff,  233 

—  rhyolite,  223 

Normal           sedimentary 

—  wacke,  233 

—  trachyte,  223 

rocks,  275 

Phosphoric    acid,    detec- 

Quartzite, 293 

Northampton  sand,  280 

tion  of,  145 

Quartzose  dacite,  234 

North  Elmsley,  Canada, 
moroxite  of,  147 

Picrite,  265 
Pilot  Knob,  Missouri,  156 

Quartzless  diabase,  245 
—    hornblende    andesite, 

Nosean,  112 

Piperno,  233 

235 

—  and  hauyne,  232 

Pisolite,  1  8,  303 

—  trachyte,  223 

Nose-pieces,  52 

Pitchstone,  195 

—  devitrification  of,  198 

—  felsitic,  197 

RAIN,  effect  of,  upon 

OBJECT-glasses,  48 
Obsidian,  186 

—  trachytic,  195 
Plagioclase,  91 

rocks,  29 
Rammelsberg,  150 

—  crystallites  in,  187 

—  basalt,  253 

Ramsay,   A.  C.,  36,   212, 

—  spherulites  in,  188 

—  enstatite  rocks,  251 

302 

Old  red  conglomerate,  302 

Plagioclastic  felspars,  91 

Rath,  G.  vom,  108,  250 

sandstone,  277 

Pliny,  203,  217 

Reiss  and  Fritsch,  233 

Oligoclase,  99 

Plutonic     and     volcanic 

Renard,   A.,  59,  86,  135, 

—  diorite,  241 

rocks,  33 

142,  165,  280 

Olivine,  116 

Pocket  lens,  44 

Reusch,  149 

—  alteration  of,  264,  271 

Polarisation  of  light,  75 

Rhombic    system,   cleav- 

— basalt,  253 

Polarising  apparatus,  48, 

ages  in,  172- 

—  gabbro,  249 

75 

Rhyolite,  178 

Oolite,  286 

Poole  clay,  284 

Richthofen,Von,  178,  193, 

Oolitic  limestones,  286 

Porcelain  jasper,  290 

237 

Opacite,  166 

Porphyrite,  237 

Roberts,  W.  Chandler,  258 

Optical  axes,  78 
—  properties  of  minerals, 

Porphyritic    structure   in 
vitreous  rocks,  185 

Rodwell,  G.  F.,  267 
Rosenbusch,  H.,  108,  142, 

74 

Porphyroid,  296 

143,  144,  148,  152,  159, 

determination 

Portland  cement,  285 

160,  167,  169,  170,  171, 

of,  79 

—  oolites,  286 

196,   199,    2ig,    221,    228, 

Orthoclase,  92 

Potstone,  271 

232,   245,  249,  254,   264 

Orthoclastic  felspars,  91 

Poussin,  Ch.  de  la  Valee, 

Rosenbusch's  microscope, 

Osteocolla,  302 

280 

54 

—  and  Renard,  135 
Practical  value  of  petro- 

Roth,  J.,  178,  234 
Rocche    Rosse,   obsidian 

PACHUCHA,  Mexico, 
tridymite  from,  152 

logical  research,  3 
Pre-Cambrian  basalts,  262 

of,  184,  189,  191 
Rock  salt,  304 

Palagonite  rock,  272 

Preliminary  examination 

in  fluid  inclosures, 

—  luff,  272 

of  rocks,  44 

165 

Paragonite,  135 

Propylite,  237 

Rochdale  flags,  278 

Paranthine,  in 

Protogine      gneiss,     212, 

Roches  moutonnees,  305 

Pearlite,  192 

295 

Rocks,    condition    under 

Pearlstone,  192 

—  granite,  213 

which  formed,  6 

Pele's  hair,  186 

Provisional  names  applied 

—  general  characters  of,  6 

Pelitic  rocks,  299 

to  minerals,  166 

Rotheram  stone,  278 

Penck,  A.,  268 
Pennant  grit,  278 
Penrhyn  quarries,  283 

Psammitic  rocks,  299 
Psephitic  rocks,  299 
Pseudo-chrysolite,  187 

Rothliegende,  278 
Rothweil,  analcime  from, 
160 

Penrith  sandstone,  279 
Percy,  J.,  285 

Pseudomorphs,  30 
Puddingstone,  301 

Roto-mahana,  33,  303 
Rounded  crystals,  178 

Perlite,  192 

Pyrites,  156 

Rounding  of  stones,  16 

Index. 


RUT                                            TAB                                            VER 

Rutile,  147 

Silurian  slates,  283 

Tachylyte,  199 

—  in  quartz,  151 

Sinter,  siliceous,    19,  33, 

Talc,  137 

3°3 

—  schist,  298 

Skiddaw  slates,  283 

Taupo     Lake,     siliceous 

SAINT      Bees      sand- 

Skye,   hypersthenite   of, 

sinter  of,  303 

stone,  279 

251 

Tawney,  E.  B.,  262 

Salt,  crystals  of,  in  fluid 

Slabs,  282 

Tertiary  limestones,  289 

inclosures,  165 

Slates,  17,  282 

—  sandstones,  281 

Sands,  17 

—  Cambrian,  283 

Tetragonal  system,  cleav- 

Sand, volcanic,  267 

—  Silurian,  283 

ages  in,  171 

Sandberger,  254 
Sand-rock,  19,  280 

Slaty  cleavage,  cause  of,35 
Slicing    rocks    with    dia- 

Thames, mud  of,  285 
Thermal  springs,  33 

Sandstones,  17,  276 

mond  dust,  63 

Thickness  of  beds,  mea- 

— Cambrian,  276 
—  carboniferous,  278 

Slievenalargy,    tachylyte 
of,  200 

surement  of,  24 
—  of  the  earth's  crust,  10 

—  cretaceous,  280 

Smith,  Lawrence,  micro- 

Tin stone,  148 

—  Devonian,  277 

scope  of,  59 

Tintagel  Quarries,  284 

—  Jurassic,  280 

Sphene,  140 

Titaniferous  iron,  154 

—  old  red,  277 

Sodalite,  112 

Titanite,  140 

—  oolitic,  280 

—  of  Somma,  116 

Tivbli,  travertine  of,  303 

—  Silurian,  276 

Sorby,  H.  C,  2,  36,  107, 

Tolcsva,  obsidian  of,  181 

—  tertiary,  281 
Sandwich   Islands,    lavas 

142,  151,  224 
Some,   Isle   of  Mull,   ta- 

Topaz, 142 
Topley,  W.,  32,  281,  289 

of,  199 

chylyte  of,  202 

Tourmaline,  137 

Sanidine,  94 

South    Burgess,   Canada, 

Trachy-phonolite,  279 

'  —  rhyolite,  224 

moroxite  of,  147 

Trachyte,  221 

•  --  trachyte,  224 

Specimens,  collection  of, 

—  proper,  225 

Scapolite,  in 

39 

Trachytes,     classification 

Scenery,  on  what  depen- 

— dressing  of,  41 

of,  222 

dent,  ii 
Schalstein,  248 

Specular  iron,  155 
Spherulites    in    obsidian. 

Trachytic  pitchstone,  195 
Travertine,  18,  303 

Scheerer,  108 

1  88 

Tremolite,  131 

Schistose  rocks,  293 

Spherulitic  structure,  183 

Triassic  sandstones,  279 

Schmidt's  goniometer,  53 

Spilite,  247 

Trichites,  162,  185 

Schorl,  138 

Stache,  237 

Triclinic   '  system,  cleav- 

— rock,  265,  298 

Statuary  marble,  287 

ages  in,  172 

—  schist,  298 

Staurolite  slate,  292 

Tridymite,  152 

Scotch  slates,  284 

StaUroscope,  81 

Tripoli,  281 

Scrope,    G.    Poulett,    37, 

Stauroscopic        examina- 

Tschermak, 88,  95,  103 

228 

tion,  84 

Tufa,  calcareous,  18,  302 

Sedimentary  matter,  sort- 

Stelzner, 171 

Tuff",  greenstone,  249 

ing  of,  in  water,  16 
—  rocks,  274 

Stockwerksporphyr,  211 
Stone  inclosures,  165 

•  —  phonolite,  233 
Twinning  of  calcspar,  149 

classification  of,  19 

Strata,  flexure  of,  21 

felspars,  87,  102 

—  denned,  15 

Stratigraphical  breaks,  31 

Seifersdorf,    minette     of, 

Streak  of  minerals,  45 

219 

Striations  in  labradorite, 

T  TNCONFORMITY, 

Seismology,  9 

102 

\^J      Stratigraphical,  31 

Semi-granite,  211 

Strike,  term  denned,  21 

United  States,   propylite 

Serpentine,  269 

Structures  in  basalt,  &c., 

of,  237 

Sericite,  134 

due  to  contraction,  14, 

rhyolite  of,  185 

-  —  •  schist,  296 

259 

Setton,  les,  152 

vitreous  rocks,  180 

Shales,  17,  282 
Sharp,  D.,  36 

Structural  planes,  12 
Subsidence  and  elevation 

T  7ARIOLITE,  248 
V      Vibration,  principal 

Silica,  globular  condition 

of  land,  ii 

directions    of,   in  crys- 

of, 152 

Syenite,  203,  217 

tals,  58 

—  percentage  of  in  erup- 
tive rocks,  177 

Syenitic  gneiss,  295 
—  granite,  203 

Vitreous  rocks,  177 
—  —  phenomena  effusion 

Siliceous  breccia,  19 
—  limestone,  20 

Synclinal,  term  defined,  22 
Szabo,  loo 

in,  179 
Ve'lain,  M.,  152 

—  sinter,  19,  33,  303 

Veltlin,  hypersthenite  of, 

Silurian  flags,  283 

251 

—  limestones,  285 

HP  ABLE    of  cleavages 

Verde,  antique  porphyry, 

—  sandstones,  276 

J.      in  minerals,  171 

240 

Index. 


319 


VES                                             WOL 

Vesuvian,  142 
—  lavas,  257 
Vesuvius,  leucitophyrs  of, 

WA  D  S  W  O  R  T  H, 
U.S.,   rhyolite  of, 
185 

256 

Ward,  J.  C.,  34,  144,  249, 

—  nepheline  of,  107 
Viridite,  166 

291 
Washing  of  fine  deposits, 

Vitreous  eruptive  rocks, 

73 

177 
Vogelsang,  H.,   113,  119, 

Watcombe  clay,  284 
Wealden  marbles,  289 

142,  163,  171,  197,  201 

Weathering  of  rocks,  29 

Volcanic  phenomena,  38 

Weiss-stein,  211 

—  and  plutonic  rocks,  de- 

Welsh slates,  283 

fined,  33 

Wernerite,  in 

—  ashes,  267 

Whin  Sill,  258 

—  bombs,  267 

Wickersley  stone,  278 

—  ejectamenta,  266 

Witham,   H.,  sections  of 

—  sand,  267 

fossils     first      prepared 

Volcanoes,   general    cha- 
racters of,  37 

Wolf  rock,  107 

Vom  Rath,  152 

tridymite  in,  152 

Vulcanicity,  9 

Wolff,  Th.,  171,  266 

ZWI 


Wollaston's  prism,  49 
Woodward,  H.  B.,  282 
Woolwich    and    Reading 
clays,  284 


\7ORKSHIRE    flags, 
1       278 

—  Jurassic  sandstones  of, 
280 


yEOLITES,  158 

/ *     Zircon,  143 

Zirkel,  F.,  106,  107,  io1, 
114,  121,  125,  129,  130, 
146,  147,  160,  166,  167, 
168,  184,  197,  201,  219, 

221,  228,  237,  249,  251 

Zwitter  rock,  211 


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